universidad autónoma de nuevo león faculty of chemical sciences

Transcription

UNIVERSIDAD AUTÓNOMA DE NUEVO LEÓN
FACULTY OF CHEMICAL SCIENCES
INFLUENCE OF FUNCTIONAL GROUPS OF ACTIVATED CARBON IN THE ANCHORING
OF IRON PARTICLES BY FORCED HYDROLYSIS AND ITS USE TO REMOVE
HEXAVALENT CHROMIUM FROM AQUEOUS SOLUTIONS
By
JONATHAN VALENTÍN REYES
As partial requirement to obtain the Degree of
MASTER OF SCIENCE with orientation in sustainable processes.
June, 2014
FACULTAD DE CIENCIAS QUÍMICAS
INFLUENCE OF FUNCTIONAL GROUPS OF ACTIVATED CARBON IN THE ANCHORING
OF IRON PARTICLES BY FORCED HYDROLYSIS AND ITS USE TO REMOVE
THESIS AUTHORIZATION:
Advisor: Dr. Refugio Bernardo García Reyes.
Co-Advisor: Dr. Ángel Martínez Hernández.
Reviewer: Dr. Eduardo Soto Regalado.
Reviewer: Dra. María Teresa Garza González.
ii
ACKNOWLEDGEMENTS
My most humble and sincere thanks to:
My sage advisor Dr. Bernardo Refugio García Reyes, for his pitch-perfect knowledge on the
matter, for allowing me the opportunity to share a same objective, for his dedication and
talent, but mainly for his endless good humor as well as his appreciated relationship.
The bright Dr. Ángel Martínez Hernández for expertly navigating more conversations, for
his skills and intelligence that helped me to understand several important things and for
spending so much time with me to comply this project.
The entire adsorption team, Dr. Eduardo Soto Regalado and Dr. Felipe de Jesús Cerino
Córdoba for their interesting classes and knowledge, and with very special thanks to Dr. María
Teresa Garza González for her grace, patient, effort and her available to hear also her help and
comments to improve this thesis.
The wise and enthusiastic Dr. Jacobo Ruiz Váldez, for the special services provided in
connection with other laboratories, support and good cheer and also for his available to hear.
The clever M.C. Adriana Liñan and all the specialist or technicians who generously spent
time with me, sharing their expertise and to the many others whom I have failed to mention in
this abbreviated list, my sincere thanks.
For all their knowledge, advises, energy, skills and help,
I am eternally grateful.
iii
DEDICATIONS
I dedicate this thesis to the most important engine of my life, my family that have allowed
me to grow up together with them living unforgettable moments, and because in one way or
another they have contributed to reach this achievement.
As always, first and foremost, my Mother Consuelo Reyes Cruz for all her love, patient,
unconditional help, and for all her sage advises that gave me in each stage of the life and
without whom all things descend into chaos, and also to my father Juan Valencia Avelino who
shared all the effort and knowledge with the family and because he always gave me his help
when I most needed.
To my sisters: Brindis Guadalupe and Celita Rodriguez, because they provide an important
part of the happiness in my life and by their outstanding councils to never give up in all my
purposes and objectives now and in the future.
To my loved children, Niurces Michelle and Carlos Uriel, who fulfill my heart of endless
love and my brain and spirit with deeply admiration, and because they always give me relax
and funny moments to forget many stressful situations.
All my irreplaceable and close friends: Laura Nájera, Dulce Zapata, Israel Torres, Omar
Rodríguez, Roberto Lira, Carolina Ramirez with whom I shared and spent time during this life
project and who were there when I really needed. Finally, but not less important, to the
incredible people that I knew during this research work: Ángeles García, Carina Sáenz,
Alejandra Sánchez and Eder Aguiñaga with whom I shared experiences, excellent
conversations, a friendship, and for helping me in the strenuous experimental work and to all
those who even are not mentioned here helped me to comply this dream.
Jonathan Valentin Reyes
iv
ABSTRACT
Jonathan Valentin Reyes
July, 2014
Universidad Autónoma de Nuevo León
Facultad de Ciencias Químicas
Dissertation title: INFLUENCE OF FUNCTIONAL GROUPS OF ACTIVATED CARBON IN THE
ANCHORING OF IRON PARTICLES BY FORCED HYDROLYSIS AND ITS USE TO REMOVE
Number of pages: 137
Candidate for the Master Degree of Sciences
with Orientation in Sustainable Processes
Study area: Sustainable Processes.
Purpose and method of study: Heavy metals are a serious health problem, because
when they are ingested can cause allergies, kidney or neuronal damage, cancer and death
in some cases; therefore, they should be removed from the effluent before discharging to
the environment. Conventional methods for removing heavy metals, such as chemical
precipitation, coagulation-flocculation and ultrafiltration, among others become inefficient
for solutions with concentrations less than 100 mg/l. In this work, the adsorption of Cr (VI)
by modified activated carbon with anchored iron particles by forced hydrolysis was studied
at different concentrations between 10 and 500 mg/l, pH 6 and 25oC as an alternative
instead of the traditional methods.
Contribution and Conclusions: Surface chemistry of the activated carbon had an
important effect on the anchoring of iron particles by forced hydrolysis, showing the higher
quantity of iron over activated carbon surface the adsorbents that were modified by an
oxidation with nitric acid, and the lower quantity of iron particles those adsorbents who
were treated thermally. However, forced hydrolysis treatment caused an important
improvement in the hexavalent chromium adsorption capacity of modified activated
carbons at concentrations lower than 150 mg/l. Heat-treated activated carbons and
modified with forced hydrolysis showed the best Cr(VI) uptake (57 mg/g) and also the
faster adsorption kinetic of the studied adsorbents. A 98% of Cr(VI) desorption from the
best adsorbent was achieved with a 0.1 N NaOH-NaCl solution, suggesting an anionic
adsorption mechanism.
ADVISOR SIGNATURE: ________________________________________
Dr. Refugio Bernardo García Reyes
v
TABLE OF CONTENTS
Chapter
Page
THESIS AUTHORIZATION ................................................................................................... ii
ACKNOWLEDGEMENTS .................................................................................................... iii
DEDICATIONS ................................................................................................................... iv
ABSTRACT ......................................................................................................................... v
LIST OF TABLES.................................................................................................................. x
LIST OF FIGURES...............................................................................................................xii
LIST OF SYMBOLS .............................................................................................................xv
1. WATER IN THE ENVIRONMENT……………………………………...……………………………………………..1
1. INTRODUCTION ............................................................................................................ 2
1.1 Water pollution with heavy metals ....................................................................... 4
2. CHROMIUM UPTAKE BY ACTIVATED CARBON………………..………………………………………………7
2.1 Chromium .................................................................................................................. 8
2.1.1 Chromium chemistry ........................................................................................ 10
2.1.1.1 Trivalent Chromium: Cr(III) .................................................................... 13
2.1.1.2 Hexavalent Chromium: Cr(VI) ................................................................ 14
vi
Chapter
Page
2.1.2 Toxic effects ...................................................................................................... 16
2.1.2.1 Absorption ............................................................................................. 16
2.1.2.2 Distribution and warehousing ............................................................... 17
2.1.2.3 Excretion ................................................................................................ 18
2.1.3 Adverse effects ................................................................................................. 18
2.1.3.1 Chromium (III) ........................................................................................ 19
2.1.3.1 Chromium (VI) ........................................................................................ 19
2.1.4 Permissible limits: Water.................................................................................. 21
2.1.5 Technologies developed for removing chromium from water ........................ 22
2.2 Adsorption processes ............................................................................................... 24
2.2.1 Activated alumina ............................................................................................. 29
2.2.2 Polymeric resins................................................................................................ 31
2.2.3 Activated carbon .............................................................................................. 33
2.2.1.1 Activated carbon treatments ................................................................. 42
2.3 Forced Hydrolysis...................................................................................................... 45
2.4 Motivation of this research ...................................................................................... 48
2.5 Hypothesis ................................................................................................................ 49
2.6 Objectives ................................................................................................................. 49
2.6.1 General Objective ............................................................................................. 49
2.6.2 Specific objectives ............................................................................................ 49
3. MATERIALS & METHODOLOGY………………..………………..……………………..…………………………50
3.1 Techniques used to characterize the activated carbon ............................................ 51
3.1.1 Boehm titration method................................................................................... 51
3.1.1.1 Boehm titration procedure .................................................................... 52
3.1.1.2 Determining the quantity of surface functional groups ........................ 53
3.1.2 Elemental Analysis ............................................................................................ 55
3.1.3 Fourier Transform Infrared Spectrometry (FT-IR) ............................................ 55
3.1.5 Nitrogen physisorption studies ........................................................................ 56
3.2 Techniques to determine metal species ................................................................... 59
3.2.1 Hexavalent chromium determination .............................................................. 59
3.2.1.1 Colorimetry ............................................................................................ 60
3.2.1.2 Ultraviolet-visible spectroscopy ............................................................ 61
vii
Chapter
Page
3.2.2 Total Chromium determination........................................................................ 63
3.2.2.1 Atomic Absorption Spectrometry .......................................................... 63
3.2.3 Trivalent chromium determination .................................................................. 64
3.2.4 Iron determination ........................................................................................... 64
3.2.4.1 Atomic absorption spectrometry (AAS) ................................................. 64
3.3 Modifications of commercial activated carbon ........................................................ 65
3.3.1 Oxidation Treatment ........................................................................................ 65
3.3.2 Thermal Treatment........................................................................................... 65
3.3.3 Nitrogen treatment .......................................................................................... 66
3.3.4 Forced hydrolysis .............................................................................................. 67
3.4 Adsorption and desorption of chromium by activated carbon ................................ 67
3.4.1 Equilibrium adsorption studies......................................................................... 68
3.4.2 Adsorption kinetics of metals ........................................................................... 69
3.4.3.1 Empirical models.................................................................................... 69
3.4.3.2 Diffusive models .................................................................................... 70
3.4.4 Desorption studies............................................................................................ 71
3.5 Residues disposal ...................................................................................................... 71
4. RESULTS & DISCUSSION……………………..…………..………………………..…………………………………72
4.1 Characterization of adsorbent .................................................................................. 73
4.1.1 Acid and basic sites: Boehm Method ............................................................... 73
4.1.2 Elemental analysis ............................................................................................ 77
4.1.3 Fourier Transform Infrared spectrometry (FT-IR) ............................................ 81
4.1.4 Physical adsorption of Nitrogen ....................................................................... 88
4.2 Batch adsorption-desorption test............................................................................. 93
4.2.1 Cr(VI) adsorption test ....................................................................................... 93
4.2.1.1 Adsorption isotherms: Commercial activated carbon ........................... 93
4.2.1.2 Adsorption isotherms: Oxidized activated carbons with HNO3 ............. 95
4.2.1.3 Adsorption isotherms: Activated carbons with thermal treatment ...... 97
4.2.1.4 Adsorption isotherms: Activated carbon with nitrogen treatment ...... 99
viii
Chapter
Page
4.2.2 Cr(VI) reduction analysis ............................................................................... 100
4.2.2.1 Adsorption isotherms: Commercial activated carbon ........................ 101
4.2.2.2 Adsorption isotherms: Oxidized activated carbons with HNO3 .......... 102
4.2.2.3 Adsorption isotherms: Activated carbons with thermal treatment .... 104
4.2.2.4 Adsorption isotherms: Activated carbon with nitrogen treatment .... 106
4.2.2.5 General discussion: Hexavalent chromium adsorption ....................... 107
4.2.3 Cr(VI) desorption test ................................................................................... 112
4.2.4 Cr(VI) adsorption kinetics ............................................................................. 114
5. CONCLUSIONS………………………..………………..………………………..……………………………………..116
REFERENCES………………………..………………..………………………..………………………………………………….120
ix
LIST OF TABLES
TABLE
PAGE
1. Source and toxicity of heavy metals on human health. ..................................... 5
2. Main industrial sources that discharge heavy metals. ....................................... 6
3. Chemical chromium species in the environment. ............................................ 12
4. Maximum permissible levels of chromium in water. ....................................... 22
5. Methods for removing metal ions from wastewater. ...................................... 23
6. Active forces in the three interfaces involved in the adsorption processes. ... 26
7. Salt of aluminum for the production of activated alumina. ............................. 29
8. Analysis of the main adsorbents used in the Cr(VI) removal. .......................... 31
9. Classification of pores size according to the IUPAC. ........................................ 33
10. Treatments applied to modify activated carbon chemistry surface. ............... 44
11. Adsorption isotherm models. ........................................................................... 68
12. Empirical models of adsorption kinetics. ......................................................... 69
13. Diffusive adsorption kinetic models. ................................................................ 70
14. Acid and basic sites of the studied activated carbon (Boehm method). ......... 73
15. Overview of elemental analysis of the studied adsorbents. ............................ 77
16. IR Assignments of functional groups on carbon surface. ................................. 82
17. IR Assignments of functional groups on carbon surfaces with forced
hydrolysis....................................................................................................... 85
x
TABLE
PAGE
18. Surface area and pore size distribution of the adsorbent materials................ 88
19. Parameter of the fitting to Freundlich for adsorption isotherms. ................... 93
20. Parameter of the fitting to Freundlich for adsorption isotherms (HNO3). ....... 96
21. Parameter of the fitting to Freundlich for adsorption isotherms (Thermal). .. 98
22. Parameter of the fitting to Freundlich for adsorption isotherms (Nitrogen). 100
23. Cr(VI) adsorption capacity for different adsorbent materials. .......................107
24. Cr(VI) adsorption capacity for activated carbons oxidized with minerals acids..109
25. Cr(VI) adsorption capacity for activated carbons modified by thermal
treatment. ...................................................................................................110
26. Cr(VI) adsorption capacity for activated carbons modified by ammonia
treatment. ...................................................................................................111
27. Adsorption kinetic parameters. ......................................................................115
xi
LIST OF FIGURES
FIGURE
PAGE
1. Pourbaix diagram for chromium species........................................................ 11
2. Species of chromium (III) in aqueous solution. .............................................. 13
3. Chromium (VI) species in aqueous solution. .................................................. 15
4. Frost diagram for chromium species in acid solutions................................... 16
5. Scheme of adsorption process by electrical forces. ....................................... 25
6. Polymeric matrices and functional groups of ion exchange resins. ............... 32
7. Scheme of chemical surface of activated carbon........................................... 37
8. Chemisorbed acid groups on the edges of the layer of graphene. ................ 38
9. Dissociation of acidic functional groups on activated carbon. ...................... 39
10. Possible basic groups in activated carbon. .................................................... 40
11. Types of nitrogen groups (a) and (d) amide groups, (b) tertiary amines, (c)
lactams, (e) type pyridine – pyrrole, (f) nitriles. .................................... 41
12. Pathway for the synthesis of iron hydro (oxides) nanoparticles ................... 47
13. Mechanism of oxidation of aromatic hydrocarbons: (a) 9, 10
Dihydrophenanthrene, (b) Diphenylmethane, (c) Benzene. ................. 79
14. Representative FT-IR spectra of the untreated carbon and modified samples
......................................................................................................................... 81
xii
FIGURE
PAGE
15. Adsorption isotherm of Cr(VI) on activated carbon (2 mg CA/mL, pH = 6 and
temperature of 25oC); Freundlich fit (continuous line ):  AC raw,  AC
raw HF. ...................................................................................................... 94
16. Adsorption isotherms of Cr(VI) on activated carbon (2 mg CA/mL, pH = 6
and temperature of 25oC); Freundlich fit (continuous line):  AC raw, 
AC Ox(10%),  AC Ox(20%),  AC Ox(10%) FH,  AC Ox(20%) FH. ....... 95
17. Adsorption isotherms of Cr(VI) on activated carbon (2 mg CA/mL, pH = 6,and
T800,  AC T900,  AC T800 FH,  AC T900 FH. ................................... 97
18. Adsorption isotherms of Cr(VI) on activated carbon (2 mg CA/ml, pH = 6,
NT,  AC NT FH. ....................................................................................... 99
19. Adsorption isotherms of Cr(VI) on activated carbon (2 mg CA/mL, pH = 6 and
T = 25oC):  AC raw Cr(VI),  AC raw total Cr,  AC raw FH Cr(VI), 
AC raw FH total Cr. ..................................................................................101
T = 25oC):  AC raw Cr(VI),  AC raw total Cr,  AC Ox(10%) Cr(VI),  AC
Ox(10%) total Cr, ▲ AC Ox(20%) Cr(VI), ▲ AC Ox(20%) total Cr. .........102
T = 25oC):  AC raw FH Cr(VI),  AC raw FH total Cr,  AC Ox(10%) FH
Cr(VI),  AC Ox(10%) FH total Cr, ▲ AC Ox(20%) FH Cr(VI), ▲ AC
Ox(20%) FH total Cr. ................................................................................103
Temperature of 25oC):  AC raw Cr(VI),  AC raw total Cr,  AC T800
Cr(VI),  AC T800 total Cr, ▲ AC T900 Cr(VI), ▲ AC T900 total Cr. .....104
xiii
FIGURE
PAGE
Temperature of 25oC):  AC raw Cr(VI),  AC raw total Cr,  AC T800 FH
Cr(VI),  AC T800 FH total Cr, ▲ AC T900 FH Cr(VI), ▲ AC T900 FH
total Cr.....................................................................................................105
Temperature of 25oC):  AC raw Cr(VI),  AC raw total Cr,  AC NT
Cr(VI),  AC NT total Cr, ▲ AC NT FH Cr(VI), ▲ AC NT FH total Cr. ......106
25. Histogram of the adsorption-desorption analysis of the different adsorbent
materials:  Adsorption capacity (mg/g),  Desorption capacity (mg/g);
Initial concentration: 30 mg/L Cr (VI), pH = 6. ........................................113
26. Adsorption kinetics analysis of the different selected adsorbent materials:
 AC raw, ▲ AC T800,  AC T800 FH. Initial concentration: 30 mg/L Cr
(VI). .........................................................................................................114
xiv
LIST OF SYMBOLS
Symbol Significance
AC
Activated carbon
Cf
Final concentration
Co
Initial concentration
et al.
And co-workers
G
Gram
GFO
Granular Ferric Oxides
K
Freundlich constant
Ke
Equilibrium constant
L
Liter
M
Mass
Mg
Milligram
oC
Celsius degrees
ppm
Parts per million
q
Adsorption capacity
qmax
Maximum adsorption capacity
rpm
Revolutions per minute
T
Temperature
V
Volume
xv
MASTER OF SCIENCE WITH ORIENTATION ON SUSTAINABLE PROCESSES
CHAPTER 1
Introduction: Water in the environment
“It isn't that they can't see the solution.
It is that they can't see the problem.”
G.K. Chesterton
1. INTRODUCTION
Water is essential for supporting life and it´s also considered as natural resource.
Because this liquid covers around 72% of earth, its pollution is an important topic about the
main environment conflicts in the world.
Due to the extensive formation of hydrogen bonds, water has high melting and boiling
points as well as large heat capacity. Additionally, water has an exceptional ability to dissolve a
wide variety of ionic and polar covalent substances because of its high polar character [Brown
(2004)]. However, taking into account that water is a universal solvent, this property allows its
use in a wide number of applications, for this reason the use of water must be done in a
sustainable way to satisfy the social and economical development as well as its preservation
(National Council Water, 2012).
It was reported that human body contains about 65% of water by mass and this high
percentage play a major role in human´s body functioning. In fact, every cell and organ
functions depends on the water for their correct functioning. Among all usages of the water, it
is also used as a lubricant in digestion and almost all the other body processes; for example,
water lubricates joints and cartilage in human body. In addition, water is capable of removing
toxins and it helps to control body temperature and it can transport valuable nutrients to the
organism.
2
It is important to mention that human body not only depends on water to subsist, this
resource is also essential for biosphere, flora, and fauna. In general, water is required for living
organisms and contributes in nutrient flows to keep the ecosystems.
Water is a renewable resource, it could be contaminated by human activities which in
turn make water to be useless or harmful. For this reason clean water is an essential resource
for the operation and development of a stable and prosperous society (UNESCO, 2003).
Many researchers have reported that water quality has been affected by factors or
agents that causes its pollution such as pathogens, inorganic and organic substances,
biodegradable wastes that require oxygen to be oxidized, sediments or suspended material,
radioactive substances and heat. Also wastewater may contain nutrients, that can stimulate the
growth of aquatics plants, and it could also contain toxic compounds with high mutagenic or
carcinogenic potential.
For these reasons, the immediate removal of pollutants from wastewater sources
(through its specialized treatment, reuse, or disposal in the ambient) is necessary for the
protection of human health and, at the same time the environment (Metcalf & Eddy, 2004).
Prevention of water pollution plays a major role to improve water quality and to reduce the
requirement of expensive water or wastewater treatments.
3
1.1 Water pollution with heavy metals
The presence of heavy metal in aqueous effluents is of great concern due to its toxicity
and carcinogenic effects on human health and aquatic organism (Prabhakaran et al., 2009).
Heavy metals are released into the environment through natural sources from volcanic
eruptions and anthropogenic sources such as wastewater discharges from industrial sources,
these pollutants are not easily removed without an advanced or specialized treatment (Leyva
et al., 2008).
Heavy metals are hazardous because of its toxic nature; furthermore they are not
biodegradables and they can also be bioaccumulated in organisms. In other words, because of
their persistence in the nature, metals can be dispersed in water, accumulated in plants and
animals, and finally to be added in human beings throughout the food chain or by drinking
polluted water, causing several health problems (García & Rangel, 2010).
Metal ions like chromium, mercury, cadmium, nickel, arsenic, lead, etc., have an
adverse effect on the human physiology and other important biological systems; for example,
the presence of heavy metals in water bodies have important toxic effects in animals and at
higher trophic levels. Because of the wide amount of processed material the removal of metal
ions from aqueous solutions (especially in low concentrations) from industrial effluents of high
interest (Liu et al., 2007). The source and toxicity of certain metal ions are listed in Table 1.
4
Table 1. Source and toxicity of heavy metals on human health (Farooq et al., 2010).
Metal
Source
Toxic effects
Reference
Lead
Electroplating, production of
batteries, pigments,
ammunitions.
Anemia, brain damage,
anorexia, loss of appetite.
Gaballah & Kibertus,
1998; Low et al.,
2000; Volesky, 1993.
Cadmium
Electroplating, enamel,
pigments production, plastics,
mining, refinery.
Carcinogenic, kidney
damage, boney injury,
hypertension, loss of
appetite.
Sharma, 1995; Chen &
Chao, 1998; Godt et
al., 2006; Singh et al.,
2006.
Mercury
Mercury wear areas, volcanic
eruptions, Forrest fires from
nature causes, battery
production, burning fossil fuels,
mining and metallurgy.
Renal and neurological
damage, spoilage to the lung
function, corrosive to eyes,
skin and muscles, dermatitis,
liver damage.
Morel et al., 1998;
Boening, 2000;
Manohar et al., 2002.
Chromium
(VI)
Electroplating, leather tanning,
textiles, metallurgy, wood
preservation, paints and
pigments.
Carcinogenic, mutagenic,
teratogenic, epigastric pain,
nausea, vomiting, severe
diarrhea, lung tumors.
Dupont & Guillon,
2003; Kobya, 2004;
Granados & Serrano,
2009; Singh et al.,
2009.
Arsenic
Enamels, mining, energy
production from fossil fuels,
sediment rocks.
Gastrointestinal symptoms,
damage to cardiovascular
and nervous system,
melanosis in bones,
hemolysis, polyneuropathy,
encephalopathy, liver
tumor.
Chilvers & Peterson,
1987; Dudka &
Market, 1992;
Robertson, 1989.
Copper
Electronics coating, galvanizing,
paints production, printing,
wood preservation.
Develops acute toxicity,
neurotoxicity, sleep, and
diarrhea.
Yu et al., 2000; Chuah
et al., 2005;
Papandreou et al.,
2007.
Non-ferrous materials, mineral
processing, formulation of
paints, electroplating, coated
porcelain, thermoelectric.
Chronic bronchitis, reduced
lung function, lung cancer.
Akhtar et al., 2004;
Ozturk, 2007.
Nickel
5
In recent years, the levels of toxic metals in surface waters have been increasing due to
pollution caused by wastewater discharges from industrial sources (Leyva et al., 2008).
Untreated effluents from industries such as metallurgy, electronics, leather tanning, plating and
cooling water systems among others, can pollute water bodies with heavy metals (García et al.,
2009).
Table 2 shows the most common industrial processes that discharge aqueous effluents
with high content of heavy metals to the environment or water bodies, and the wide use of
chromium in many industries.
Table 2. Main industrial sources that discharge heavy metals (Mohan et al., 2006).
Industrial Source
Zn
Automobiles
X
Petroleum refining
X
Pulp and paper
X
As
X
Textile
Cr
Pb
Ni
Cu
X
X
X
X
X
X
X
X
X
X
X
X
Fe
Hg
X
Cd
Sn
X
X
X
X
X
X
X
Steel
X
X
X
X
Organic chemicals
X
X
X
X
X
X
X
Inorganic chemicals
X
X
X
X
X
X
X
Leather tanning
Mining
Glass
X
X
X
X
X
6
X
X
CHAPTER 2
Background: Chromium (VI) uptake by activated carbon
“The man who has ceased to learn ought not
to be allowed to wander around loose in
these dangerous days”
M.M. Coady
2.1 Chromium
Chromium (Cr) is a naturally occurring element found in rocks, plants, soil, animals as
well as in smoke and volcanic gases; it is the sixth most abundant transition metal in the Earth´s
crust, where it is found like oxide (Cr2O3) and, combined with iron and oxygen in the mineral
chromite. Chromium was discovered in 1797 by the French chemist Lois Vauquelin in lead
chromate (PbCrO4), which is the rare mineral crocoite; its name comes from the Greek
“chromos” meaning “color” and it is due to the different colors found in their compounds
(Mohan et al., 2006).
Chromium, with an atomic number of 24 and atomic weight of 51.996, is a heavy metal of the
first series of transition metals (Group VI B). Chromium has important chemical and
biochemical properties, among those include several oxidation states; some chromium
compounds are paramagnetic and most of them are colorful, therefore some mineral and
precious stones attribute their color to chromium (Zayed et al., 2003).
Chromium oxidation states are -2, 0, +2, +3, and +6, but the only compounds of biological
importance are derived from the oxidation states of +3 and +6; the first group belong the
chromic oxide (Cr2O3) and chromic salts such as chromic chloride (CrCl3) or chromite anion
(Cr(OH)4-), and the second group, the chromium trioxide (CrO3), chromate (CrO4)2-, and
dichromate (Cr2O7)2- (ATSDR, 2008).
The most important mineral form of chromium is called chromite ore or ferrochromite
(FeCr2O4) derived from chromium (III).
8
On the other hand, metallic chromium (Cr0) is not free in nature due to its high reactivity, it is
white silver bluish tint, exhibiting the property of being heat resistant and easily coated with a
thin layer of oxide protecting it from further chemical reaction. Metallic chromium is higher
resistant to oxidation, even at high temperatures. This property, combined with its colors, is
the reason of being used as coat of different metal objects in order to protect against
corrosion, processes known as chrome plating (Mohan et al., 2006; Klassen et al., 2008).
Compounds derived from chromium (III) are usually prevailing in the environment because they
are the most stables derivate from this metallic element. The most important compound from
trivalent chromium is chromic oxide (Cr2O3) that is extremely stable, resistant to acids and also
high melting point. It is used such a pigment called “chrome green”.
In contrast, chromium (VI) derivatives are fundamentally chromates and dichromates. Both
species are oxidizing agents in acidic media, and they are reduced to Cr(III) by electron donors
(Kuo et al., 2008). Potassium dichromate is frequently used in industry and chemical
operations. Sodium dichromate is used in leather tanning since an insoluble compound is
produced with skin proteins. Often, lead dichromate is used as pigment with the name
“chrome yellow”.
Chromium is mainly used in three industries: metallurgic, chemical and refractory materials. In
the metallurgical industry is an essential compound of stainless steel and other metal alloys.
9
Frequent uses in refractory materials include brick manufacturing based on a mixture of
magnesite-chromium for metallurgic furnace and the use of granular chromite for many other
applications requiring heat resistant materials.
Regarding applications in chemical industries, it is used in operations of chromium plating,
manufacture of pigments and dyes, leather tanning and wood treatments. Minor amounts of
these chromium compounds are used as corrosion inhibitors, water treatment, photographic
materials, magnetic tapes, etc. (Mohan et al., 2006; Klassen et al., 2008; Leyva et al., 2008).
2.1.1 Chromium chemistry
Chemical speciation analysis of chromium, and other elements, provides information about
individual concentration of this particular element in the different forms in which it is present
in biosphere. This analysis not only allows distinguish among oxidation states, also between
simple and coordinate ions, cationic, neutral and anionic forms, ionized and not ionized species
as well as the degree of homogeneity and heterogeneity with natural constituents (Kotás &
Stasicka, 2000).
Among the chromium oxidation states, only two of these, trivalent and hexavalent chromium,
are enough stable to be in the environment. However, these forms are drastically different in
charge, physicochemical properties as well as in chemical and biochemical reactivity.
10
Moreover, chromium in its divalent form, Cr(II), is quickly oxidized to Cr(III), and it remains
stable as Cr(II) only in the absence of any oxidizing agent (i.e. under anaerobic conditions). The
species Cr(IV) and Cr(V) form unstable intermediates in the reactions with the oxidation states
of trivalent and hexavalent chromium and oxidizing and reducing agents, respectively (Palmer,
1994; Mohan et al., 2006; Kotás & Stasicka, 2000).
Pourbaix diagram illustrated in Figure 1 shows the pH and potential conditions where each
chromium species is thermodynamically stable in dilute aqueous solutions, in the presence of
air and absence of any complex agent that are not H2O or OH-.
Figure 1. Pourbaix diagram for chromium species (Palmer et al., 1994).
11
A general summary of chromium species (0 – VI) and the presence of some compounds of them
in the nature are shown in the Table 3.
Table 3. Chemical chromium species in the environment (Zayed et al., 2003).
Chemical
Oxidation
specie
state
Elemental
chromium
Cr(0)
Divalent
Cr(II)
chromium
Trivalent
Cr(III)
chromium
Compounds
Observations
It is not naturally present in the environment.
CrBr2, CrCl2,
Relatively unstable and easily oxidized to trivalent
CrF2, CrSe, Cr2Si
state.
CrB, CrB2, CrBr3,
Form stable compounds and are present in nature in
CrCl3.6H2O,
the minerals as ferrochromite (FeCr2O4).
CrCl3, CrF3, CrN
Tetravalent
Cr(IV)
CrO2, CrF4
chromium
It is not present in nature and it is an important
intermediate which affects the rate of reduction of
Cr(VI). Compounds of Cr(IV) are less common. The
Cr(IV) ions and their compounds are not very stables
due to their short half-lives, forming intermediates en
the reactions of the species of Cr(VI) and Cr(III).
Pentavalent
Cr(V)
chromium
Hexavalent
chromium
Cr(VI)
CrO43-,
It is not present in nature and it is an important
potassium
intermediate which affects the rate of reduction of
percromate
Cr(VI). Species of Cr(V) are derivate from CrO43- anion.
(NH4)2CrO4,
Second most stable state of chromium oxidation.
BaCrO4 ,CaCrO4,
However, the Cr(VI) is difficult to be in nature, but it
K2CrO4 , K2Cr2O7
is produced by anthropogenic sources. Nevertheless,
it is present in the atmosphere in the rare mineral
crocoite (PbCrO4).
12
2.1.1.1 Trivalent Chromium: Cr(III)
Trivalent chromium species in the environment depends on the different chemical and physical
processes, such as hydrolysis, complex formation, redox reactions and adsorption. Oxidation
state of Cr(III) is the most stable and requires a substantial amount of energy to be converted
to a state of lower o higher oxidation.
Hydrolysis of Cr(III) produce a mononuclear species CrOH2+, Cr(OH)2+, Cr(OH)4-, neutral species
Cr(OH)3 and polynuclear species Cr2(OH)24+ , Cr3(OH)45+. All these trivalent chromium species
that prevail in aqueous solutions and its dependence on pH value are shown in Figure 2.
Figure 2. Species of chromium (III) in aqueous solution (García et al., 2009).
13
Hydroxo complexes (CrOH2+ y Cr(OH)3) are the major species of Cr(III) in the aquatic
environment. However, the Cr(OH)3 exhibit amphoteric behavior and at high pH values can be
easily transformed into a complex tetrahydroxoion (Cr(OH)4- ) (Rai et al., 1987).
The Cr(III) is a strong acid which has a high tendency to form six-coordinated octahedral
complexes with a wide variety of ligands such as water, ammonia, urea, ethylamine, and other
organic compounds containing oxygen, nitrogen or sulfur.
Trivalent chromium complexation with ligands as OH- increases its solubility when the ligands
are in the form of discrete molecules or ions. Because the redox potential between Cr(VI) and
Cr((III) is sufficiently high, just a few naturally occurring oxidants are able to oxidize Cr(III) to
Cr(VI). Schroeder & Lee (1975) reported that oxidation of Cr(III) by dissolved oxygen in
aqueous solutions is not possible. However, Saleh et.al. 1989 reported that through manganese
oxides, an effective oxidation could be achieved in environmental systems.
2.1.1.2 Hexavalent Chromium: Cr(VI)
Hexavalent chromium in aqueous solutions forms a number of species and the relative
proportions of these species depends on both pH and concentration of Cr(VI). The influence of
the pH on Cr(VI) speciation is shown below in Figure 3. Hydrolysis of Cr(VI) produces only
neutral and anionic species, predominantly H2CrO4,CrO42-, HCrO42- and Cr2O72- (Mohan et al.,
2006; Dionex, 1996).
14
Figure 3. Chromium (VI) species in aqueous solution (García et al., 2009).
Chromic acid (H2CrO4), the neutral specie of Cr(VI), undergoes two deprotonating steps
according to equations (1.1 to 1.3) forming the HCrO4- ion (pH between 1 y 6) and the CrO42- ion
(pH greater than 7).
𝐻2 𝐶𝑟𝑂4 ↔ 𝐻 + + 𝐻𝐶𝑟𝑂4−
𝐾 = 10−0.75
𝐻𝐶𝑟𝑂4− ↔ 𝐻 + + 𝐶𝑟𝑂42−
𝐾 = 10−6.45
2𝐻𝐶𝑟𝑂4− ↔ 𝐶𝑟2 𝑂72− + 𝐻2 𝑂
𝐾 = 102.2
(1.1)
(1.2)
(1.3)
Most of Cr(VI) species are highly soluble anions and for this reason, Cr(VI) species can be
transported through the soils and water. Nonetheless, the oxyanions of Cr(VI) are easily
reduced to Cr(III) by electron-donors atoms or molecules like organic materials or inorganic
reduced species that could be present in air, water and soils (Pyrzynska, 2012).
15
The hexavalent chromium in acid solutions shows a high positive redox potential (Eo between
1.33 – 1.38 V, Figure 4), which denotes that it is strongly oxidizing and unstable in presence of
electron donors (Mohan et al., 2006; Kotás & Stasicka, 2000).
Figure 4. Frost diagram for chromium species in acid solutions (Kotás & Stasicka, 2000).
2.1.2 Toxic effects
2.1.2.1 Absorption
Chromium can enter in the human organism by inhalation, ingestion and, in lesser quantities,
by absorption through the skin. Furthermore, it may be absorbed through the respiratory tract
when inhaled air containing this metallic element and it takes place in the lungs through cell
membranes of the alveoli.
16
Chromium absorption occurs via the digestive tract as a result of the ingestion of food or water
containing any derivative from this metal, and exhibits significant effects when it is in contact
with gastric juice. In the gastrointestinal tract of human and animals, Cr(III) is absorbed less
than 1% while Cr(VI) around 10% (Klassen et al., 2008).
Trivalent chromium present in food can bind to other compounds that facilitate its absorption
in the stomach and intestines. Zhitkovich in 2005 conducted studies on Cr(VI) absorption, and
reported that once absorbed, Cr(VI) enters the cells and it is intracellular reduced to Cr(III) as
effect of red blood cells essentially cause by glutathione, ascorbic acid, and cysteine (ATSDR,
2008; USA EPA).
2.1.2.2 Distribution and warehousing
Chromium fraction absorbed in the intestine is transported into the blood, where it is
distributed to the different organs. Once it has been absorbed, chromium (III) does not easily
pass into cell membranes, but binds to transferrin (plasma protein which transport iron).
In contrast, after Cr(VI) absorption, it travel quickly to erythrocytes where it is converted to
Cr(III). Protein complexes of trivalent chromium are deposited mainly in the bone marrow,
lungs, lymph nodes, spleen, kidney, and liver (USA EPA).
17
2.1.2.3 Excretion
Inhaled particles containing elemental chromium can be retained in the lungs several years.
Chromium excretion occurs mainly through the urine. In humans, kidney excretes
approximately 60% of absorbed Cr(VI). Sedman et.al 2006, reported that the half-life for the
excretion of potassium dichromate is about 35-40 hours. It is estimated that 10% of the
absorbed dose of chromium is removed by biliary excretion and lower quantities are removed
through hair, nails, milk, and sweat. As soon as the chromium was ingested with water and
food, most Cr is eliminated with feces.
2.1.3 Adverse effects
The effects of chromium on human health mainly depend on the valence state at the time of
exposure as well as its solubility. The most important toxicological forms are chromium (VI) and
chromium (III) (Pyrzynska, 2012). Hexavalent chromium compounds, strong oxidizing agents,
tend to be corrosive and irritating pollutants. In addition, Cr(VI) species are considerably more
toxic than chromium (III) compounds, if doses and solubility are similar. It is postulated that this
different may be related to the easy manner that the hexavalent chromium cross through cell
membranes and subsequent intracellular reduction to reactive intermediates (ATSDR, 2000;
Klassen et al., 2008).
18
2.1.3.1 Chromium (III)
Trivalent chromium is an essential nutrient for the metabolism of sugar and fats, and enhances
insulin action as part of the glucose tolerance factor. Chromium deficiency in the human body
is not frequent, most cases are seen in undernourished or diabetic people. Chromium
deficiency is characterized by glucose intolerance, hypercholestoremia, reduced longevity,
reduced sperm count, fertility disorders, weight loss, reduced growth and nervous system
dysfunction (García & Rangel, 2010; Miretzky et al., 2010).
However, prolonged exposure to excessive quantities of chromium (III) can cause health risks,
among them, human body can develop a higher sensibility to chromium causing skin redness
and injuries (USA EPA).
2.1.3.1 Chromium (VI)
As above-mentioned, the most important toxic effects of chromium are mainly attributed to
the hexavalent state. Chronical exposure of eyes to hexavalent chromium species is observed
as conjunctivitis, tearing and pain. On the other hand, chromic acid and its salts are corrosive to
skin and mucous membranes which can cause severe damage such as chronic ulceration and
perforation of the nasal septum. The characteristic injury caused by a random dermal exposure
to hexavalent chromium compounds is a sharp and deep ulcer that not ooze and its recovering
is slow (ATSDR, 2000; WHO, 1996).
19
In addition, exposure to hexavalent chromium compounds can cause allergic reactions
characterized by severe redness and edema of skin. On the other hand, long occupational
exposure of the respiratory system to high levels of Cr(VI) in inhaled air (more than 2 mg/m 3)
may cause irritation of nose, bleeding, ulcers, etc. (Klassen et al., 2008).
Inhalation for short periods of small quantities of chromium (VI) into the air, does not cause
adverse effects in most people, however, in allergic people to this metal, exposure to high
concentration of chromium in air can trigger asthma attacks and other diseases like rhinitis,
laryngitis, nosebleeds, nasal pain, pulmonary fibrosis, etc. (ATSDR, 2008).
Ingestion of small quantities of hexavalent chromium has no adverse effects on the
gastrointestinal tract. However, the accidental or intentional ingestion of large quantities of
Cr(VI) causes an acute gastrointestinal pain with bloody vomiting, diarrhea, blood in the feces
and can cause cardiovascular collapse. If the intoxicate person survives, subsequent effects
may be liver and kidney necrosis and, perhaps death (Klassen et al., 2008; Demiral et al., 2008).
Currently, occupational exposure to hexavalent chromium compounds, especially in the
production of chromium and pigments industries, are associated with an increased risk of lung
cancer, given that is considered as cancergenic and genotoxic compound (Miretzky et al. 2010;
Saha et al., 2010).
20
Once hexavalent chromium enters to cells, it is reduced by various intracellular agents to give
reactive species as trivalent chromium. During this reduction process, different genetic
damages can be produced like damage to DNA structures. Reduction of Cr(VI) can also generate
oxygen radicals, inhibits protein synthesis and stop DNA replication (WHO, 1996).
O´Brien et al. (2003) reported that all these effects could play an integral function in the
chromium carcinogenicity. Inhaled compounds of Cr(VI) can penetrate many body tissues and,
therefore they have the potential to cause lung or cancer at different sites. Costa and Klein in
2006 indicated that could be a close relationship among Cr(VI) and cancer of prostate, stomach,
kidney, urinary system, and bones cancer. Similarly, Seedman et al. 2006 reported that there is
a relationship between stomach cancer and hexavalent chromium present in drinking water
(Klassen et al., 2008).
2.1.4 Permissible limits: Water
Because of the toxic effects of chromium, it is important to regulate the discharge of this
pollutant in receiving water bodies. There are different agencies worldwide (namely WHO, EPA,
SEMARNAT) responsible for establishing and enforcing environmental regulations. Mexican
regulations establish that industrial waste effluents containing chromium are considered
hazardous and must be disposed of or treated appropriately according to the norm NOM-052SEMARNAT-2005.
21
The table 4 shows the maximum permissible levels for chromium in effluent discharges into
diverse water bodies according to the actual regulations.
Table 4. Maximum permissible levels of chromium in water established by different regulations.
Maximum
Environmental regulation
permissible limit
Description
(mg/L)
Sets the maximum permissible levels of pollutants
NOM-001-SEMARNAT-1996
0.50 - 1.00
in discharges in sewage waters and national
properties.
Sets the maximum permissible levels of pollutants
NOM-002-SEMARNAT-1996
0.50
in discharges of wastewater to urban or municipal
sewage systems.
Establishes the maximum permissible levels for
NOM-127-SSA1-1994
0.05
drinking water and treatments that must be applied
for such purpose water purification.
WHO
0.05
EPA
0.10
Guidelines for drinking-water quality.
U.S EPA 40 CFR Parts 141, 142 and 143, National
Primary Drinking Water Regulations.
2.1.5 Technologies developed for removing chromium from water
Different treatment technologies have been developed for the removal of chromium from
wastewater, in order to comply with the maximum permissible limits according to the
applicable regulation and to reduce the toxicological effects of chromium after effluent
discharge in water bodies.
22
The most common methods of treating wastewater contaminated with heavy metals include
chemical precipitation, ionic exchange, membrane separation, electrocoagulation, chemical
reduction and/or oxidation, reverse osmosis, evaporation recovery processes as well as the
adsorption processes, among others (Blázquez et al., 2009; Singha et al., 2011; Ertugay et al.,
2008). Table 5 shows a comparative study of conventional methods used for the removal of
heavy metals from wastewater.
Table 5. Methods for removing metal ions from wastewater (Farroq et al., 2010).
Method
Chemical precipitation
Chemical coagulation
Advantages
 Simple
 Inexpensive
 Most metals can be removed


Ionic exchange
Sludge sedimentation
Dehydration
 Excellent regeneration of
materials
 Selective
Disadvantages
 Large amounts of sludge is
generated
 Waste disposal problems

High cost

High consumption of
chemical reagents
 Fewer ions removed
 High capital costs
Adsorption
By activated carbon


By zeolites
 Most of cationic metals can be
adsorbed
 Relatively less cost of materials

Ultrafiltration y
Membrane processes
Most of metals can be adsorbed
High efficiency (>99%)


Lower production of solid waste
Lower consumption of chemical
reagents
High efficiency (>95% for a single
metal)
23


Cost of activated carbon
Dependence on the type
of adsorbent
 Low efficiency



High initial capital and
operating costs
Low flow rates
Percentage of removal
decreases with the
presence of other metals
The most frequently used method for control of discharges of Cr(VI), after a reduction
processes, is the chemical precipitation method, but have disadvantages such as poor
selectivity, continuous consumption of chemicals and sludge production increasing the total
cost of disposal of solid waste (Kumar et al. 2011).
In general, these treatment processes have considerable limitations such as incomplete metal
removal, requirements for expensive equipment and monitoring systems, high reagent and
energy requirements, and generation of toxic sludge or other waste product requiring final
disposal (Ertugay et al., 2008).
Furthermore, the ion exchange is a better way for the treatment of heavy metals containing
effluents; however, ion exchange is not economically viable due to high cost of the polymeric
resins compared to adsorption processes, in which the price of activated carbon in less than
polymeric resins (Jung et al., 2013; Leyva, 2007).
2.2 Adsorption processes
The adsorption phenomenon is a mass transfer operation that allows the selective removal of a
pollutant (adsorbate) present in a fluid (aqueous o gas) on a solid surface (adsorbent) that has a
high affinity for the adsorbate (Metcalf & Eddy, 2004).
24
A number of adsorbent materials have been tried for metal ions removal including activated
carbon, zeolites, activated alumina, synthetic polymers and silica gel among others, but the last
three adsorbents are less common used in adsorption processes because of their high cost
(Metcalf & Eddy, 2004).
Moreover, the adsorption is defined as the concentration or accumulation of a solute on the
surface of a solid, caused by disequilibrium of the surface forces. On the other hand, adsorption
can be defined as a process where atoms in a solid surface, attract or retain molecules of other
compounds by electrostatic forces as depicted in Figure 5; these attraction forces are known as
Van der Waals forces (Crittenden et al., 2005).
Figure 5. Scheme of adsorption process by electrical forces.
Taking into account that adsorption phenomenon occurs on the surface of the adsorbent, high
surface area would enable the better performance of the adsorbent. Nevertheless, the
adsorption process is also strongly influenced by the functional groups present in the
adsorbent surface.
25
Other forces involved in the adsorption process are dipole-dipole and dipole-cuadropole
interactions, London or van der Waals dispersion forces, covalent bonding and hydrogen bond
formation as shown in Table 6 (Coneey, 1998; Critteden et al., 2005).
Table 6. Active forces in the three interfaces involved in the adsorption processes.
Approximate
Force
Interface
Energy of
Interaction,
kJ/mole
Adsorbate/
Adsorbate/
Water/
adsorbent
Water
adsorbent
Coulombic repulsion
>42
Yes
No
No
Coulombic attraction
>42
Yes
No
No
Yes
No
No
Ionic species - neutral species
attraction
Covalent bonding
>42
Yes
No
No
Dipole – Ion species attraction
<8
Yes
Yes
Yes
Dipole –dipole attraction
<8
Yes
Yes
Yes
Dipole – Induced dipole attraction
<8
Yes
Yes
Yes
Hydrogen bonding
8 – 42
Yes
Yes
Yes
Van der Waal´s attraction
8 – 42
Yes
Yes
Yes
Adsorption is classified depending on chemical or physical interaction type between the
adsorbate and adsorbent surface. Physical adsorption is a reversible phenomenon which
results from intermolecular forces of weak electrostatic attraction between complexes from
solid surface and molecules of the adsorbate. The molecules do not adsorb on a specific site on
the surface and they can move freely in the interfase.
26
This kind of adsorption occurs at low temperatures and is characterized by an exothermic heat
very similar to condensation heat (Leyva, 2007; Bottaniand et al., 2008). Chemical adsorption is
due to a chemical interaction between the specific surface complexes of the adsorbent and
adsorbate molecules, typically occurs at high temperatures (> 200oC) and a high activation
energy (10-100 Kcal/gmole); involves formation of chemical bonds and, generally, it is
irreversible. Additionally, the adsorption heat is very high and similar to the heat of chemical
reaction (Leyva, 2007; Cooney, 1998).
Adsorption in liquid phase is caused by interactions between solute in solution and functional
groups on the surface of the solid adsorbent. The main factors affecting adsorption are the
following (Leyva, 2007; Metcalf & Eddy, 2004; Crittenden et al., 2005; Cooney, 1998):
a) Texture properties of the adsorbent such as specific surface area, average pore
diameter and pore volume, physical and chemical properties of the adsorbent such as
surface charge distribution, which depends on the type and the quantity of functional
groups on the adsorbent surface; and the chemical composition of the adsorbent,
among others.
b) The chemical and physical characteristics of the adsorbate, for example: molecule size,
polarity, solubility, chemical composition and concentration of adsorbate in solution.
c) The characteristics of the liquid phase, such as pH, temperature, ionic strength and
polarity.
27
Adsorption has evolved as a cost effective method for removing chromium. Among the most
commonly used adsorbents for removing chromium from aqueous solutions are activated
carbon and polymeric resins.
Activated carbon and polymer resins have a high capacity for removing metal ions from
aqueous solution, and activated carbon present a superior chemical stability than other
adsorbents materials (zeolites, metal oxides and biosorbentes such as chitin, chitosan,
seaweed, etc.) (Fang et al., 2007). Special characteristics of activated carbon like high surface
area, pore structure, high adsorption capacity, functional groups and high chemical stability
make it a versatile adsorbent. In addition, saturated activated carbon can be regenerated and
reused once again and the recovered pollutant can be return to the process where it was
generated.
Otherwise, it has been reported the use of metal oxides to remove Cr(VI) from aqueous effluents
and they have shown to be highly efficient; however, this adsorbent cannot be used in continuous
systems because of the high pressure drop in packed bed columns and, additionally, separation of
metal oxide adsorbents used in batch systems is not easy due to the small particle size (Arrigo et
al., (2010).
More recent applications of metal oxides as adsorbents include anchorage of metal oxide
particles in porous materials by different impregnation techniques, for example: chemical
precipitation (Vitela et al., 2013), incipient wet impregnation (Kuo & Bembenek, 2008), synthesis
processes in-situ and ex-situ (Cannon et al., 2008) as well as forced hydrolysis processes (Sarkar
et al. 2012).
28
2.2.1 Activated alumina
Activated alumina is a semi-crystalline inorganic porous material constituted essentially by
aluminum oxide. It is a compound that present a high surface area, mechanical strength and
amphoteric character, aluminum oxides are found in several crystalline forms. The principal
phase of the activated alumina used as adsorbent is the gamma alumina (γ − Al2 O3).
Activated alumina is prepared from aluminum salts (Table 7) through many processes. It is very
important to properly control the process of preparation and activation since the products of
the reaction can be multiple in term of crystalline structure and adsorption capacity (Singha et
al., 2011).
Table 7. Salt of aluminum for the production of activated alumina (Naiya et al., 2009).
Mineral name
Chemical composition
Crystallographic Designation
Gibbsite
Aluminum trihydroxide
𝛾- Al(OH)3
Bayerite
𝛼- Al(OH)3
Nordstrandite
Al(OH)3
Boehmite
Aluminum hydro(oxide)
𝛾-AlOOH
Diaspore
𝛼- AlOOH
Pseudoboehmite
Al2O3(1.3-1.5)H2O
Aluminum oxide
𝛼 - Al2O3
Corundum
29
Activated alumina can be prepared by two methods: i) thermal dehydrolixation of aluminum
hydroxides and ii) thermolysis at elevated temperatures of hydrated aluminum salts.
Hydroxides and oxyhydroxides of aluminum are generally precursor substances to prepare
activated alumina with a great adsorption capacity (Leyva, 2007). The basic method of
preparation of γ − Al2 O3 consist of a dehydration of hydrated aluminum oxides at relatively
low temperatures (300 to 700oC).
The values of specific area and diameter of pores of the activated alumina area are from 150 to
500 m2/g and from 3 to 12 nm, respectively, and these depend on the preparation method. An
elemental analysis of the activated alumina reveals that is constituted basically by aluminum
oxide (Al2O3), and it can contain impurities like Na2O, Fe2O3 and SiO2 (Leyva, 2007).
Activated alumina have a wide variety of industrial applications such as gas desiccant, catalysts
support and catalyst in different chemical reactions (hydrodesulfuration, cracking, reforming
and isomerization of hydrocarbons of petroleum, among others). In addition, activated alumina
is a very effective adsorbent to remove certain trace elements present in gases and liquids. Due
to the pHpzc of the activated alumina around 6.2 to 8.9, it is mainly employed for the removal of
anions (arsenate [HAsO4-2], fluoride [F-], phosphate [PO4-3], and selenate [SeO4-2]) present in
aqueous solutions. The main applications of activated alumina are the removal of arsenate and
fluoride anions from drinking water for human consumption (Swarupa et al., 2006; Tingzhi et
al. 2008).
30
2.2.2 Polymeric resins
Many macroreticular commercial polymers are based on synthetic polymers such as styrene
and acrylic ester, among others. These polymeric matrices are characterized by the presence of
functional groups that contain O, N, S and P (Figure 6) which are able to form complexes with
metal ions. This special characteristic gives the polymeric resins properties of being used as ion
exchangers (Pilsniak et al., 2007). Ion exchange resins are used for water treatment since they
can be easily regenerated and the adsorbed chemical pollutants can be recovered. In some
industrial fields, the polymeric resins are used because they can be designed for specific
separations (Annesini et al., 2007).
Particularly, ion exchange resins contain selective functional groups which are very useful to
ion separation of noble metals from aqueous solutions (Pilniak et al., 2007). Currently, there is
a raising interest in polymeric porous resins as especial adsorbent for the removal of volatile
organic compounds from contaminated gas streams (Liu et al., 2009; Long et al., 2013). Table 8
shows a comparison of the main adsorbents used to remove Cr(VI) from aqueous solutions.
Table 8. Analysis of the main adsorbents used in the hexavalent chromium removal.
Adsorbent
pH
% Removal
Q (mg/g)
Reference
Activated alumina
2
97.44
-
Bishnoi et al., 2004
Granular activated alumina
4
99.00
7.44
Mor et al., 2007
IRN77 resins
2.75
95.00
35.38
Rengeraj et al., 2001
Amberlite resin
6.9
-
52
Bhatti et al., 2013
Activated rice husk carbon
2
93.28
-
Bishnoi et al. (2004)
Activated charcoal
2
99.00
12.87
Mor et al. (2007)
31
Figure 6. Polymeric matrices and functional groups of ion exchange resins [García et al. (2009)].
32
2.2.3 Activated carbon
Activated carbon (AC) is a material having a lattice crystal structure similar to graphite. It has a
highly porous surface with small amounts of heteroatoms of oxygen, hydrogen and nitrogen
which are chemically bonded. Carbon is the principal constituent of activated carbon making up
between 85% and 95% in weight of the adsorbent material. Also, AC exhibits a wide range of pore
sizes, classified as shown in Table 9, from visible cracks until openings of molecular dimension
cracks [Bottaniand & Tascón (2008)].
Table 9. Classification of pores size according to the IUPAC [Schultz et al. (1999)].
Type of pore
Pore diameter, d (nm)
Macropores
Mesoporos
𝑑 > 50
2 ≤ 𝑑 ≤ 50
Micropores
𝑑<2
Ultramicropores
𝑑 < 0.7
Supermicropores
0.7 ≤ 𝑑𝑜 ≤ 2
Note: do is the pore width, for pore type slot; d is the diameter of pore, for cylindrical pores.
Activated carbon is known as an effective adsorbent due to its high developed porosity, high
surface area (up to 3000 m2/g), variable characteristics of chemical surface and a high degree of
surface reactivity [Elizondo (2009)]. Currently, activated carbon is widely used as catalyst,
catalytic support and adsorbent. Activated carbon as adsorbent can be used for the removal of a
wide variety of species like organic substances, metal ions, and other pollutants in liquid or
gaseous phase as well as it is used in purification and recovery of chemical species [Elizondo
(2009), Dias et al. (2007)].
33
Activated carbon presents some advantages compared with other adsorbents such as silica,
zeolites and porous polymers. Generally, activated carbons are cheap, heat and radiation
resistant, stable in acid and basic solutions; they show good mechanical resistance, not expand
or contract by effects of changes in pH and AC adsorbents are economically effective from the
point of view that can be regenerated, while the silica are degraded at high pH values whereas
zeolites are dissolved at acidic pH values and they are usually rather efficient and expensive in
terms of regeneration because of its small size of pore structure. In addition, porous polymers
structures show contractions or expansion by pH variation and the cost of these adsorbents is
higher than activated carbons [Yang et al. (2007), Yue et al. (2009)].
Adsorption in aqueous phase of organic and inorganic compounds has been one of the most
important applications of the activated carbon. Rivera-Utrilla et al. (2001) reported that about
80% of total produced activated carbon is used in liquid phase applications. When activated
carbon is contacted with an aqueous solution, an electric charge is released. This charge is result
of the dissociation of functional groups attached to the surface of activated carbon or by ions
adsorption from solution, and the electric charge strongly depends on pH of solution and
adsorbent surface characteristic [Li et al. (2002)].
The central topic of ions adsorption in aqueous medium is the understanding of the mechanism
by which ionic species begin to adhere onto carbon surface. There are three different
mechanisms by which metal ions (or other ions) are removed from aqueous solutions [Dias et al.
(2007)].
34
The first mechanism is based on electrostatic interactions between adsorbate and adsorbent
being totally dependent on the existence of functional groups on carbon surface, especially
oxygenated surface complexes (ion exchange process). The second mechanism suggests an
improvement in the potential for adsorption, as what happened due to the narrowness of
microposority which can be strongly sufficient to adsorb and retain ions. The third mechanism is
based on the concept of acid and bases, strong and weak, consequence of the amphoteric
nature of the carbon surface [Elizondo (2009), Dias et al. (2007)].
Natural and synthetic precursors have been used for manufacturing activated carbon.
Carbonaceous natural precursors consist of wood, coal, lignite, peat, coconut shells, rice husks,
bones, sawdust, etc., while synthetic precursors include polymeric materials such as nylon,
rayon, cellulose, phenolic resins, polyacrylonitrile resins, etc. [García et al. (2009)].
Activated carbon manufacturing begins with pyrolityc carbonization of raw precursor materials.
During carbonization step many of the non-carbon elements (such as oxygen, hydrogen and
nitrogen) are eliminated as volatile products in the pyrolitic decomposition of the precursor and
the remaining residues promote the formation of graphite. Residual atoms of carbon are staked
in a system of flat sheets of aromatic rings that are united randomly [Leyva (2007), Elizondo
(2009)].
35
This arrangement of sheets is irregular and therefore leave interstitial spaces between them.
Pores are formed from these interstitial spaces. In the carbonization step, these spaces can be
filled with tarred materials or products of pyrolysis and they can be blocked by free carbon
atoms. At the same time or after carbonization, an activation step is required to develop a highly
porous structure [Leyva (2007)].
Activation of carbon may be physical or chemical. Physical activation, is also called thermal
activation, involves the carbonization at 500-600 °C to eliminate most of the volatile matter
followed by a partial gasification between 800 °C and 1000 °C in the presence of an oxidizing gas
(steam, carbon dioxide, air or a mixture of gases) to develop porosity and to increase surface
area [Yang et al. (2007)].
Chemical activation involves the incorporation of inorganic additives, metal chlorides such as
chloride of zinc or phosphoric acid in the precursor before carbonization, this process is
performed generally at lower temperatures (400-600 °C) than physical activation. Chemical
agents helping to develop porosity in activated carbons, mainly from dehydration and
degradation of carbon surface [García et al. (2009)]. The best advantages of chemical activation
are high performance, low activation temperature (less energy costs), less activation time,
usually, develops a higher porosity, nonetheless has some disadvantages as additional cost for
activate agents and an additional washing step is also necessary for removing residual chemical
agents [Elizondo (2009)].
36
Characteristic of activated carbon depend strongly on both the precursor material as well as
activation method (activation conditions and temperature employed), among other factors.
Figure 7 depicts a scheme of chemical surface of activated carbon.
Figure 7. Scheme of chemical surface of activated carbon [Arrigo et al. (2010)]
On the surface of carbonaceous material, particular carbon atoms of the edge of the basal planes,
can be found predominantly combined in greater or lesser proportion with others atoms different
from carbon (heteroatoms), giving place to different surface groups. These activated carbon
present functional groups that can be acid or basic depending on its behavior in solution. Most of
them are oxygenated groups, due to the tendency of the carbons to be oxidized even at room
temperature. Delocalized electrons of  orbitals play an important role in the surface chemistry
of activated carbons [Sevilla (2012)].
37
The surface of activated carbon is mainly non-polar, but its surface presents the interesting and
advantageous feature to be easily modified by heteroatoms (oxygen, nitrogen, hydrogen,
phosphorus, sulfur, etc.) [Mohd et al. (2010)].
Oxygen is the most important heteroatom which forms covalent bonds with carbon; however,
carbon-oxygen bonds are less stables than carbon-hydrogen bonds. Chemisorbed groups that
contain oxygen in the edges of the graphene layers (see Figure 8), give acid properties to
activated carbon and allow the property of cation exchange.
Figure 8. Chemisorbed acid groups on the edges of the layer of graphene [Leyva (2007)].
Surface groups which carbon-oxygen bonds are quite important and have great influence on
surface properties such as: wettability, polarity and acidity besides physicochemical properties
such as catalytic, electrical and chemical reactivity.
38
It has been reported that carboxylic groups increase by chemical oxidation of activated carbon
with nitric acid or activation of AC by air at high temperature. In both cases, oxidized AC improved
the adsorption capacity of metal cations from aqueous solutions [Jaramillo et al. (2010)].
Nevertheless, an increment in the content of oxygenated functional groups can modify the porous
carbon texture because they can block some of the micropores decreasing accessibility to porous
network, while thermal treatment at high temperature eliminate surface functional groups and
cause a collapse of the porous texture diminishing the volume of pores [Sevilla (2012)].
Dissociation of acidic functional groups such as carboxylic, lactone and phenolic provoke a
negative charge on the adsorbent surface as shown in Figure 9 [Yang et al. (2007)]. These ionized
functional groups could adsorb positive charged metal cations or dyes from aqueous solutions.
Figure 9. Dissociation of acidic functional groups on activated carbon [Leyva (2007)].
39
Basic behavior of the activated carbon surface is usually caused by certain functional groups
containing oxygen atoms such as pyrone, superoxide ions (O2-) and also groups containing
nitrogen including pyridine, quaternary ammonium, nitrogen oxides, nitriles, amines, amides,
etc. [Mohd & Hossein (2010)]
Some other oxygen containing groups like chromene and diketone (Figure 10) have been
reported to contribute to the basic character of the activated carbon. On the other hand,
non-polar surface of the activated carbon has basic properties originated from 𝜋 electrons of
carbon rings.
Figure 10. Possible basic groups in activated carbon [García et al. (2009)].
Nitrogen groups that are covalently bound to the activated carbon surface (Figure 11) remain
at the AC surface after a thermal treatment at elevated temperatures (lower than 1200K).
40
Figure 11. Types of nitrogen groups (a) and (d) amide groups, (b) tertiary amines, (c) lactams,
(e) type pyridine – pyrrole, (f) nitriles [Mohd et al. (2010)].
Some basic groups (Brönsted-Lowry bases) tend to attract protons when they are found in an acid
medium. The behavior of basic groups can be used as adsorption sites of anionic species. Surface
charge of an adsorbent depends on the quantity of acid and basic functional groups. The pH at the
point of zero charge (pHPZC) from an adsorbent is acid when the concentration of acid sites is
greater than that of the basic sites and vicerversa [Leyva (2007)].
When activated carbon acquires negative net surface charge, the cation species adsorption will be
preferred, but anion pollutants adsorption is enhanced on positively-charged activated carbon.
Taking into account that effluent pH values is not always an easy task and implies an increment in
the total cost of water treatment, it is preferred to modify the surface chemistry of activated
carbon to increase the pollutants adsorption [García et al. (2009)]. For these reasons, sometimes
may be necessary to optimize the porous texture and/or incorporate functional groups on
activated carbon surface retaining particles of interest [Yang et al. (2007)].
41
2.2.1.1 Activated carbon treatments
Activated carbon is modified by several treatments to incorporate or eliminate functional
groups, and to modify its chemical surface. When the surface of an adsorbent is modified, new
functional groups are created on the AC surface and they change the physical and chemical
properties of the adsorbent. In addition, these modifications provide thermal stability, and
reactivity of the AC. It is important to mention that AC new features depend on the nature of
the modified or attached surface groups [Zhao et al. (2005); Mohd et al. (2010)].
Oxidation process is mainly used to incorporate functional groups containing oxygen on the
activated carbon surface. It is believed that oxidation reactions probably occur in the aliphatic
chains of the activated carbon surface or in peripheral carbon atoms because these sites are
highly susceptible to be oxidized. Generally, oxidation increases the acid groups and decrease
basic groups. However, Sayed et al. (2004) have been reported, as a strange case, a slight
increment in basic functional groups as well as acid functional groups by chemical oxidation of
AC with nitric acid because of incorporation of nitrogen and oxygen on the adsorbent surface.
Oxidation of the activated carbon may be categorized into two methods: dry and wet
oxidation. In Dry oxidation, the adsorbent is in contact with oxidizing gases such as steam,
carbon dioxide, oxygen, ozone and usually takes place at elevated temperatures (greater that
970 K). In wet oxidation, activated carbon reacts with a solution of oxidizing agents such as
nitric, sulfuric or a mixture of acids, under moderate concentration and temperature conditions
[Mohd et al. (2010), Rivera et al. (2011)].
42
Oxidation of activated carbon with air or oxygen can increase the ketone and phenolic groups,
or hydroxyl and carbonyl groups. Similarly, it was reported that oxidation of AC by ozone keeps
the adsorbent texture, but oxidation creates large quantities of carboxylic groups on AC
surface. Nonetheless, ozonation treatment decreases the surface area and pore volume of
activated carbon [Mohd et al. (2010), Valdés et al. (2011)].
Nitrogenation is another common treatment applied to the activated carbon to protect the
environment by the removal of sulfur compounds, nitrogen oxides and carbon dioxide; in
catalysis applications as catalyst or catalytic support; and in electrochemistry for the electrodes
manufacturing, cells and batteries [Rivera et al. (2011)]. Nitrogenation of activated carbon
increases its basicity which is strictly required in adsorption and catalytic processes. This
treatment significantly increases the surface polarity and therefore, specific interactions with
polar adsorbates are possible [Saleh et al. (2011)].
Ammonia have been used to incorporate nitrogen atoms in activated carbon, before or after
oxidation with nitric acid (amination) or a mixture of gases of air – NH3 (ammoxidation). In
addition, HCN, HNO3, urea, diocyanoamine, melanin, polyaniline have been used for
nitrogenation of activated carbon. Depending on the used reagent, the nitrogenation
treatment can be performed in gas (ammonia, cyanuric acid, amines) or liquid phase (nitric
acid, urea) [Rivera et al. (2011), Saleh et al. (2011)]
43
Generally, nitrogenation process consist of one or two consecutive stages in which the
activated carbon is oxidized first in liquid phase and later nitrogen is added to AC in liquid or
gas phase [Rivera et al. (2011)]. It has reported ammonia treatment develops surface area and
microporosity of AC. As a consequence of this treatment, nitrogen containing functional groups
are covalently bound to the activated carbon surface. Moreover as a consequence of the
nitrogenation gets forming nitrogenized functional groups on the activated carbon surface.
Table 10 shows several treatments for binding specific functional groups on activated carbon.
Table 10. Treatments applied to modify activated carbon chemistry surface.
Treatment
Oxidation
HNO3
H2O2
Air
Ozone
Heating under
hydrogen atmosphere
Nitrogenation
NH3
Modification
Reference
Increase of acid and basic groups.
Increased surface area and surface
oxygenated groups.
Increase in phenolic and ketone groups.
Increase of carboxylic groups, decreases
surface area and pore volume.
Decomposition of oxygenated groups,
increase in basicity.
Sayed et.al. (2004)
Domingo et.al. (2000)
Increase of amine, nitriles, pyridines, and
amines, lactams, and imides groups.
Vinke et.al. (1994)
Biniak et.al. (1997)
Abe et.al. (2000)
Przepiórski et.al. (2004)
Przepiórski et.al. (2006)
Stöhr et.al. (1991)
Jansen & Bekkam (1994)
Bashkova et.al. (2003)
Stavropoulos et.al. (2008)
HCN
Increase of amine and nitrile groups.
Urea
Increase of pyridine, pyrroles, N
quaternary.
44
Figuereido et.al. (2000)
Jaramillo et.al. (2002)
Agulai et.al. (2003)
Bandosz et.al. (2006)
Ríos et.al. (2007)
Table 10. Treatments applied to modify activated carbon chemistry surface (continued).
Treatment
Sulfuration
H2S
SO2
Na2S
Modification
Increase of the content of sulfur, slight
decrease in pore volume.
Decrease in surface area (BET), increase
in the average diameter of pore,
formation of bonds C=S.
Decreases the surface area, increase the
content of sulfur about 7.3% in weight,
formation of bonds C=S, S=O, S-S.
Reference
Feng et.al. (2006)
Tajar et.al. (2009)
Valenzuela et.al. (1990)
2.3 Forced Hydrolysis
Metal oxides and hydroxides (e.g. iron, zirconium, titanium, manganese, aluminum, etc.) have
been tried as adsorbents and researchers have reported high efficiencies towards anionic
species from aqueous solutions [Beker at al. (2010), Nieto et al. (2012)].
Due to the high adsorption capacity, availability and low cost, iron hydro (oxide) have been
widely used. Some researchers have synthetized with success granular ferric oxides (GFO) with
surfaces around 250 m2/g; however, there are some disadvantages for its applications
adsorbent, for instance: the mechanical resistance of GFO must be improved; high pressure
drops are reached when GFO are packed in columns and loss of adsorbent is possible in
continuous operation. On the other hand, if GFO are used in batch operations, the adsorbent
recovering is not an easy task [Music et al. (2004), Arcibar et al. (2012)].
45
An interesting option to overcome these shortcomings is the anchorage of metal hydro (oxides)
on the activated carbon surface. Methodologies used for anchoring metal oxides particles on
activated carbon include evaporation of iron salt in presence of the support, incipient wetness
impregnation at room temperature using aqueous or organic solutions; iron precipitation with
alkali solutions as well as forced hydrolysis processes of ferric salts.
Iron particles impregnation techniques allow the formation of iron hydro(oxides) nanocrystals
on the activated carbons and these anchored particles increased the adsorption capacity of
anion species such as hexavalent chromium [Nieto et al. (2012), Arcibar et al. (2012)].
Synthesis of iron oxides nanoparticles is widely described in literature. However, most of these
techniques require voluminous chemical products such as tensoactives, chelating agents, and
organometallic compounds which have a low level of diffusion in the pores of the activated
carbon. Nevertheless, among the other options, forced hydrolysis (also called thermal
hydrolysis) is a simple route that allows the synthesis of iron hydro(oxide) nanoparticles inside
the pores of the activated carbon without the use of additional chemical products [Shultz et al.
(1999), Music et al. (2004), Wang et al. (2008), Muñiz et al. (2009)].
46
In forced hydrolysis a metal cation in solution is heated promoting the metal ions
agglomeration until the formation of a solid phase on the support. Depending on the
experimental conditions for forced hydrolysis (temperature, metal concentration, time of
hydrolysis, pH, mass/volume ratio, etc.), crystalline phases and the particle size are formed on
the porous support material (Figure 12). If the iron hydrolysis is carried out in the external
surface of the activated carbon, it is expected that certain quantity of the iron hydro (oxides)
particles will link to the activated carbon surface [Shultz et al. (1999), Wang et al. (2008)].
Figure 12. Pathway for the synthesis of iron hydro (oxides) nanoparticles by forced hydrolysis
[Shultz et al. (1999)]
47
Recent studies have used the method of forced hydrolysis to bind iron oxides particles on the
activated carbon surface; however, the influence of functional groups of the activated carbon
to anchoring iron oxides nanoparticles by forced hydrolysis have not been studied as well as its
potential use as adsorbent for the removal of hexavalent chromium from aqueous solutions.
2.4 Motivation of this research
The use of activated carbon as adsorbent of metal cation pollutants in water is the preferred
method when these pollutants are present at trace concentrations levels. On the other hand,
metals oxides have a high adsorption capacity of anionic species, but they are used only once
and they are not totally recovered from the treated effluent limiting its application in real
systems because of the increment in operation cost of the process.
According to the above-mentioned reason, to immobilize iron nanoparticles onto activated
carbon is of great interest because it will allow regenerating the saturated adsorbent, to reuse
the regenerated adsorbent and to recover the adsorbate to perform the adsorption process as
an attractive economically alternative. Additionally, it is possible to reduce the environmental
impact of pollutants into receiving aquatic streams and, at the same time complying with the
federal regulations about the maximum permissible levels of pollutants.
48
2.5 Hypothesis
The activated carbon functional groups allow anchoring iron hydro(oxides) particles in the
adsorbent surface by forced hydrolysis and dispersed particles increases the Cr(VI) adsorption
capacity greater than commercial activated carbon.
2.6 Objectives
2.6.1 General Objective
To evaluate the effect of activated carbon functional groups on the anchoring of iron hydro(oxides)
particles by forced hydrolysis and to determine its potential use for the removal of hexavalent chromium
from aqueous solutions.
2.6.2 Specific objectives

To modify the activated carbon functional groups by chemical and thermal treatments.

To anchoring iron hydro(oxides) particles on raw and modified activated carbon by
forced hydrolysis.

To characterize the raw, modified and impregnated activated carbons by instrumental
(FTIR, SEM, XRD, N2 physisorption) and conventional (Boehm titrations) techniques.

To conduct adsorption-desorption experiments of Cr (VI) in batch systems using the
raw, modified, and impregnated activated carbons.

To carry out experiments adsorption kinetics of Cr(VI) in batch system with the selected
adsorbents.
49
MASTER IN SCIENCE WITH ORIENTATION IN SUSTAINABLE PROCESSES
CHAPTER 3
Materials & Methodology
“I have learned that success is to be
measured not so much by the position
that one has reached in life as by the
obstacles which he has had to overcome
while trying to succeed”
Bokker T. Washinton
3.1 Techniques used to characterize the activated carbon
3.1.1 Boehm titration method
Traditionally called “Boehm titration” have been used as chemical method to identify
oxygenated surface groups on carbon materials. Boehm titration acts over the principle which
the oxygenated groups of the activated carbon surface have different acidity and they can be
neutralized with different bases. Sodium hydroxide (NaOH) is the strongest base generally
used, and it is used to neutralize all Brönsted acids (including phenols, lactone and carboxylic
acid groups), whereas the use of sodium carbonate (Na2CO3) neutralize lactone and carboxylic
groups (lactol and lactone rings), and sodium bicarbonate (NaHCO3) solution neutralizes
carboxylic acids groups. The difference among the neutralization of the bases may be used to
identify and quantify the types of oxygenated surface groups present in the sample of activated
carbon.
For applying the Boehm method, some factors have to be taken into account, for example: the
mass of carbon and reaction base ratio, stirring time, CO2 expulsion method from solutions,
and titration method for the determination of the equivalence point [Boehm (2002)].
It was reported that CO2 expulsion is conducted through boiling (or reflux), degasification with
N2 during the reaction and titration, complete titration under a N2 atmosphere, or by
subtracting the value obtained from the titration of a blank (without carbon) of the sample
results [Kim et al. (2009), Kalijadis et al. (2011)].
51
In determining the equivalence point, several methods can be used, for instance: indicators
such as phenolphthalein, methyl red and blue methylene or pH measurements of “one point”
(titration is performed until the value of pH 7 is reached). Most of the experimental works have
reported the use of a blank in titrations. This issue removes dissimilarities which can be
achieved due to differences in the functional groups of the activated carbon as well as
properties as wettability or origin of itself [Goertzen et al. (2010)].
3.1.1.1 Boehm titration procedure
The general titration procedure is based on the method described by Boehm. A mass of 100 mg
of activated carbon was added to 25 mL of one of the three bases of reaction with
concentration 0.1 N: NaHCO3, Na2CO3, and NaOH. The samples were stirred for a week at 25oC
and after stirring the samples were filtrated to remove the activated carbon. Finally, aliquots of
10 mL from were withdrawn the filtrated samples, and they were titrated directly with 0.1 N
HCl [Goertzen et al. (2010)].
A similar procedure was performed to determine total basic groups, but an activated carbon
mass (100 mg) was added to 25 mL of 0.1 N HCl. The samples were shaken for a week at 25oC,
filtered to separate the activated carbon and subsequently the filtrate aliquots were titrated
with 0.1N NaOH [Bagreev et al. (2004)].
52
3.1.1.2 Determining the quantity of surface functional groups
The equations used to determine the quantity of surface functional groups depend on the
titration method: back-titration or direct titration. For a back-titration, the quantity of acid
functional groups is estimated:
𝑛𝐻𝐶𝑙
[B]VB
𝑛𝐵
[HCl]VHCl = [NaOH]VNaOH + (
𝑛𝐶𝑆𝐹 =
V
− 𝑛CFS ) V a
B
𝑛𝐻𝐶𝑙
VB
[B]VB − ([HCl]VHCl − [NaOH]VNaOH )
𝑛𝐵
Va
(3.1)
(3.2)
Where [B] and VB are the concentration and volume of base of reaction mixed with the
activated carbon, and 𝑛𝐻𝐶𝑙 , 𝑛𝐵 indicate the moles available of acid and base in the
neutralization reaction, respectively. These data allow to obtain the number of moles of the
base of reaction which were available to react with the functional groups of the activated
carbon. The parameter 𝑛𝐶𝑆𝐹 indicates the moles of functional groups of activated carbon that
reacted with the base during the stirred stage. Va is the volume of the aliquot which was taken
from VB, [HCl] and VHCl are the concentration and volume of acid added to the aliquot of the
original sample.
This parameter (𝑛𝐶𝑆𝐹 ) provides the numbers of equivalents added to the aliquot, and available
to react with the remaining base. The moles of the remaining acid are then determine by
titration using a certain volume (VNaOH) of known concentration ([NaOH]). In this way, through
the knowledge of the remaining moles of the acid, and according to the difference of the used
bases of reaction, the acid groups of the activated carbon can be quantified [Hu et al. (2001),
Wang et al. (2009)].
53
When NaOH is used in Boehm titration and directly titrated with HCl, the following equation is
used:
𝑛CSF = [B]VB − [HCl]VHCl
VB
Va
(3.3)
The amount of different possible surface groups (phenols, lactones, carboxylic) are quantified
by the difference in the estimated quantity of the reacted equivalents from the different bases
(𝑛CSF). Equivalents of NaOH reacts with all surface acid groups; therefore, the value 𝑛CSF will
include all surface acid groups. Because the solutions of Na2CO3 reacts with carboxylic and
lactones groups, the difference between the values 𝑛CSF from NaOH and 𝑛CSF from Na2CO3 is
equivalent to the quantity of phenolic functional groups on the activated carbon surface.
Similarly, NaHCO3 solution reacts only with carboxylic groups (for this reason, it provides a
direct measurement of carboxylic groups) and the difference between Na2CO3 and NaHCO3 is
the number of lactone functional groups on activated carbon. For blank solutions (without
activated carbon) 𝑛CSF must be equal to zero, because there are not adsorbent particles
present in the solution and, therefore, no functional groups which react with the base
equivalents [Kim et al. (2009)].
54
3.1.2 Elemental Analysis
Elemental composition (carbon, hydrogen, nitrogen, and sulfur) for the synthetized adsorbent
materials was conducted in an elemental analyzer equipment (Permin Elmer 2400) and
Thermogravimetric analysis (TA Instrument, Model SDT2960) in oxidizing atmosphere for
determining the ash content for a complete characterization of the activated carbon. Oxygen
percentage was obtained by difference of initial mass sample and the sum of the all
determinated elements plus ash content.
3.1.3 Fourier Transform Infrared Spectrometry (FT-IR)
Infrared spectrometry is a very versatile tool that is applied for qualitative determination of
molecular species. The applications of the infrared spectrometry are divided in three
categories: near, middle, and far infrared. The most commonly used region is by far the midinfrared categories which are extended from about 650 to 4,000 cm-1, being the main tool for
the determination of organic and biochemical species [Silverstein et al. (2005)].
The use of the Fourier transform for the processing of the obtained data from infrared
spectrometry has been widely used, because it offers a signal/noise ratio which exceeds that of
the dispersive instruments in more than one order of magnitude, also it is characterized by high
resolutions (<0.1 cm-1), and highly accuracy and reproducibility for the determination of the
frequencies [Skoog (2001)].
55
Infrared analysis was performed in agreement after drying and pulverizing activated carbon
samples at 100oC during 24 h. The FT-IR spectra of the samples were recorded over an interval
of 400 a 4000 cm-1 with a total of 50 measurements (Spectrum-one Perkin Elmer).
3.1.5 Nitrogen physisorption studies
The textural properties of the activated carbon are of great relevance in order to understand if
adsorbates can diffuse through the activated carbon pores. The use of nitrogen at low
temperature (77.4 K) have been traditionally proposed as a probe molecule for the
determination of major textural parameters such as specific surface area, micropores volume,
and so on, after the application of the corresponding mathematic models [Silvestre et al.
2012].
To determine the specific surface area, pore volume and average diameter of porous it is
required a physisorption equipment (Quantachrome Instruments, Autosorb-1). The basis of this
equipment is the gas (N2, Argon, or CO2) adsorption method. The specific surface area analysis
is obtained by the theory of Brunauer, Emmett y Teller (BET). The BET theory is based on the
formation of multilayers and supposes that the adsorption heat of the monolayer is different
from the rest of the other layers, which have the same adsorption heat.
56
To estimate the amount of the necessary adsorbed gas to the formation of a monolayer (Vm) is
required the use of the following equation [Sing et al. (1985), Brunauer et al. (1938)]:
(3.4)
P
1
c−1 p
=
+
V(P0 − P) Vm c Vm c p0
ε1 − ε2
c = exp {
}
RT
(3.5)
Where:
P
Pressure of N2 in equilibrium with the adsorbed gas in the adsorbent, atm.
P0
Saturation pressure of N2 at the experimental temperature, atm.
V
Volume of adsorbed N2 referred at standard pressure and temperature, m3.
Vm
Volume of adsorbed N2 referred at standard pressure and temperature which is
required to form a monolayer on the adsorbent surface, m3.
ε1
Adsorption heat of the first layer of N2, cal/mole.
ε2
Condensation heat of N2, cal/mole.
R
Ideal gas constant, cm3-atm/mole-K
T
Absolute temperature, K.
57
The specific surface area (S) of the material (BET area) is obtained once the volume of the
adsorbed gas on the monolayer (Vm) is known, as the following equation [Brunauer et al.
(1938)]:
(3.6)
S=(
Ps Vm c
) NSN2
RT0
Where:
N
Avogadro´s number, 6.023 x 1023 molecules/mole.
Ps
Standard pressure, 1 atm.
S
Specific area of the adsorbent, nm2.
SN2
Projected area which occupy a molecule of N2, 16 nm2/molecule.
T0
Standard temperature, 273.15 K.
The pores size distribution is performed in accordance with the method developed by Barret,
Joyner and Halena (BJH method), 1951. Most specifically, the method of Dubinin-Radushkevich
(DR) allows to obtain the value of micropores volume [Nguyen & Do (2001)]:
W = W0 exp (−
K
2)
β2 (RTln(P⁄P0 ))
Where:
W
Adsorbed volume to each relative pressure
W0
Volume of pores
K
Constant dependent on the structure
B
Affinity coefficient
58
(3.7)
Experimental N2 physisorption data were obtained as follows: the sample holder of the
equipment is closed with a filter seal, this holder is placed in the degasification port, and
vacuum is applied until obtaining a pressure less than 100 μm Hg. Subsequently, sample holder
was removed, weighed and activated carbon sample (approximately 120 mg) was placed at the
sample holder.
After that, the sample holder was weighed and placed in the degassing port. The sample was
degassed at 105oC until reach a vacuum less than 100 μm de Hg. Next, the sample holder
containing the activated carbon was removed from the degassing port, it was weighted and the
weight of the sample was calculated by difference. Finally, the sample holder was placed at the
test port of the equipment and the analysis was carried out automatically.
3.2 Techniques to determine metal species
3.2.1 Hexavalent chromium determination
The determination of Cr(VI) was performed as described in the Mexican Norm NMX-AA-044SCFI-2001, which is based primarily on a qualitative technique at one the principles of
colorimetry, followed by UV-visible spectroscopy at 540 nm.
59
3.2.1.1 Colorimetry
The colorimetric method is useful for the determination of hexavalent chromium in natural or
treated water in a concentration range of 100 to 1000 μg/L. The principle of this method is
based on a oxidation-reduction reaction, where the hexavalent chromium (Cr6+) reacts with 1,5diphenylcarbazide in acid solutions to give Cr3+ and 1,5-diphenylcarbazone according to the
following reaction [Clesceri et al. (1995), Dionex (1996)]
+
3+
3+
2CrO2−
+ H2 L + 8H2 O
4 + 3H4 L + 8H → Cr (HL)2 + Cr
(3.8)
Where:
H4L = 1,5-diphenylcarbazide
H2L = 1,5-diphenylcarbazone
This reaction seems to be the simultaneous oxidation of the dipheylcarbazide to
diphenylcarbazone, reduction of Cr(VI) to Cr(III), and the chelation of Cr(III) by
diphenylcarbazone.
The actual chelate structure is unknown, but it is identified by visible absorbance using a
photometric detector. To determine initial and final hexavalent chromium from aqueous
solutions, aliquots were taken according the Cr(VI) content content and the calibration curve
range (0.1 – 1 mg/L).
60
Afterwards, aliquots were transferred to volumetric flasks, and they were filled up to 50 mL
with added 0.1 N sulfuric acid. Next, one ml of diphenylcarbazide solution (5 mg/mL) was
added to each flask and they were stirred vigorously for one minute. Finally, samples were
allowed to stand for 10 min for developing the red-violet color completely and the sample
absorbance was measured at 540 nm using a spectrophotometer.
3.2.1.2 Ultraviolet-visible spectroscopy
Absorbance measurements, at both ultraviolet and visible radiation, have a wide application
for identifying and determining inorganic and organic species. The UV-Vis spectroscopy is the
an adequate technique for quantifying the Cr(VI) concentration in solution. The red-violet
complex, a product formed during the redox reaction between Cr(VI) and 1,5diphenylcarbazide, can be determined in an UV-Vis spectrophotometer at a wavelength of 540
nm by using a calibration curve.
Firstly, a calibration curve was constructed by measuring at least 5 volume of stock solution
(5.0 μg of Cr(VI)/mL) approximately between 2.0 mL and 20.0 mL for standards in the range of
10 μg to 100 μg of Cr(VI) or a concentration range of 0.1 to 1 mg/L of Cr(VI).
Next, the procedure for color development of sample was followed. Aliquots of each standard
were transferred to absorption cells of 1 cm and their absorbance was measured at 540 nm in a
spectrophotometer (GENESYS 10s UV-Vis Spectrophotometer).
61
Calibration solutions were measured starting with the lowest concentration. Finally, a
calibration curve was constructed by plotting the read absorbance vs mg/L de Cr(VI).
Before sample analysis, sample must be as clear as possible, and the colorimetric procedure of
diphenylcarbazide is performed. Then, the spectrophotometer was adjusted with a blank
(water with diphenylcarbazide) to zero and the sample absorbance was measured at 540 nm.
Finally, absorbance measurements were recorded and the mg/L of Cr(VI) present in the sample
were determined directly from the calibration curve as follows:
X=
Y−b
m
Where:
(3.9)
m
Slope of the calibration fitting curve
b
Y intercept of the calibration fitting curve
Y
Absorbance of sample at 540 nm
X
Sample concentration of Cr(VI), mg/L.
The concentration of the analyzed samples (in mg of Cr(VI/ L) was calculated with the following
equation:
mg Cr⁄L =
mg/L Cr(obtained from the calibration curve)
∗ 𝐷𝐹
A (mL of original sample)
Where DF represent the dilution factor used to analyze the hexavalent chromium.
62
(3.10)
3.2.2 Total Chromium determination
3.2.2.1 Atomic Absorption Spectrometry
Atomic absorption spectrometry (AAS) constitutes a sensible test for the quantitative
determination of more than 60 metal or metalloid elements. The method to determine metals
in natural, drinking and wastewaters by AAS is based on the generation of atoms in the basal
state and measurement of the amount of absorbed energy by them, which it is directly
proportional to the concentration of that element in the analyzed sample [Beaty (1993)].
The experimental procedure for total chromium determination was developed as indicated in
the Mexican Norm NMX-AA-051-SCFI-2001. In brief, five standards of chromium (25 mL) were
prepared in a concentration range of 0 to 20 mg/L by using a certified standard to perform the
calibration curve for subsequent samples analysis by atomic absorption spectrometer (Thermo
Jarrell Ash, Model MSCAN1).
Samples of chromium in solution were aspirated and atomized into air/acetylene flame; a
monochromatic electromagnetic radiation (𝜆 = 359.3 nm) passed directly into the flame and
the atomized sample, then a detector measured the amount of light absorbed by the atomized
chromium. The quantity of absorbed energy is proportional to the quantity of chromium in
sample and it was determined from the calibration curve.
63
3.2.3 Trivalent chromium determination
The quantity of trivalent chromium (Cr3+) was determined by the difference between the total
chromium (obtained by atomic absorption) and the hexavalent chromium (estimated by
ultraviolet-visible spectroscopy) as shown in Equation 3.11. Trivalent chromium, is an
important parameter to analyze and to determine if the hexavalent chromium was reduced
during the adsorption process, and it helps to suggest a mechanism based on adsorption/
reduction [López et al. (2012)].
μg Cr 3+ = μg Cr 0 (Atomic absorption) − μg Cr 6+ (Uv − vis)
(3.11)
3.2.4 Iron determination
3.2.4.1 Atomic absorption spectrometry (AAS)
The content of iron was determined after an acid digestion of the iron-loaded activated
carbons. A sample of activated carbon (200 mg) was powdered and placed in a 50 mL plastic
capped tube and 25 mL of a solution of HNO3/H2SO4 or HCl/H2SO4 (5:1 v/v ratio) was added.
Flask was capped and remained in constant stirring for 24 h. The activated carbon was
separated from the solution by filtration. The iron concentration of the filtrated was obtained
by atomic absorption spectrometry (Thermo Jarrell, Model MSCAN1) at a wavelength of 248.3
nm.
64
3.3 Modifications of commercial activated carbon
3.3.1 Oxidation Treatment
The activated carbon was modified in acid medium as follows: 5 grams of activated carbon
were placed in a distillation flask with 3 necks containing 150 mL of HNO3. The adsorbent
material was oxidized with solutions of 10, 20 and 40% v/v from concentrated nitric acid (min.
68% - max. 71%). A condenser was connected to this flask in the central neck and a
thermometer in the other neck. The mixture was heated to the boiling point (68 - 70oC) and
suspension was refluxed for 2 hour. After the oxidation period, the system was allowed at
room temperature. Subsequently, the remaining acid was removed from the flask and the
adsorbent material was recovered by filtration with glass filters. The oxidized activated carbon
was washed several times with deionized water or 0.01 N NaOH solutions until the mixture
adsorbent-water reached a neutral pH. Finally, modified activated carbon was dried in an oven
at 100oC for 24 hours [Andrade et al. (2007)].
3.3.2 Thermal Treatment
The removal of carboxylic groups on the surface of the activated carbon by a heat treatment
has been widely reported, resulting in a decrease in the quantity of acid sites (essentially
carboxylic and lactones groups) and an increment of basic sites cause by the formation of
pyrone structures.
65
The procedure for carrying out this modification of the activated carbon is mentioned
below: The thermal process was conducted under a nitrogen flow (60 cm3/min) and it was
started by heating, a sample of 100 mg of activated carbon, from room temperature to a
selected temperature (800, 900oC) at a heating rate of 10oC/min. Samples were maintained
at constant temperature (800, 900oC) for 2 h. Afterwards, the system was cooled under
nitrogen flow (60 cm3/min) until room temperature.
3.3.3 Nitrogen treatment
The incorporation of nitrogen containing groups on the activated carbon surface was
performed by a treatment with concentrated ammonium hydroxide. Activated carbon (5
mg/mL ratio) was placed in a sealed vessel with concentrated ammonium hydroxide,
continuously stirred at 150 rpm for 24 h. Next, the suspension was filtered into a fume
hood, and the modified activated carbon was dried at 100oC for 24 h, in order to react the
remaining ammonium hydroxide present in the sample [Wibowo et al. (2007), Mangun et
al. (2001)].
66
3.3.4 Forced hydrolysis
The granular activated carbon was modified with iron hydro (oxides) particles by forced
hydrolysis using FeCl3 ∙ 6 H2 O solutions as follows: A sample (1000 mg) of activated carbon
was placed in a glass vessel containing 50 ml of distilled water at a temperature of 100 oC for 5
min, to remove air bubbles from pores, to moisten the adsorbent material and to promote the
diffusion of iron particles inside the activated carbon pores, ultimately the solution was cooled
at room temperature. The wet activated carbon was placed in 200 mL of 1 mM FeCl3 at pH 1
adjusted with sulfuric acid.
This suspension was heated at 60oC and 200 rpm for 1.5 hours. After heating, samples were
washed with distilled water to remove all iron from the outer surface of the activated carbon.
Finally the iron loaded adsorbent was dried at 100oC for 24 h.
3.4 Adsorption and desorption of chromium by activated carbon
Adsorption isotherms of the activated carbon provide important information to determine the
adsorption capacity of the adsorbent material as well as to obtain models and kinetic
parameters which play a key role for designing and comparison purposes. However, desorption
studies are essential to regenerate the activated carbon for its subsequent use and to recover
the adsorbate for final disposal, treatment or re-use.
67
3.4.1 Equilibrium adsorption studies
A mass of activated carbon (m = 50 mg) was added to a volume (V = 25 mL) of Cr(VI) solutions
which initial concentration (Co) varied from 10 to 500 mg/L. These suspensions were adjusted
to pH 6 with 0.1 N HCl or NaOH as required, and these experiments were continuously stirred
at 200 rpm for 7 days at 25oC. Once the equilibrium was achieved, the final pH was measured
and the activated carbon was separated by filtration and aliquots were taken from the filtrate
to determine the Cr(VI) concentration. The equilibrium concentration (Ce) was determined by
the colorimetric method coupled with UV-Vis spectroscopy [Gupta el al. (2010), Kobya (2004);
Terzyk (2001)]. The adsorption capacity at equilibrium (qe) of the activated carbon was then
determined with the following equation:
(3.12)
V(Co − Ce )
qe =
m
Equilibrium data was used to calculate the parameters of the adsorption isotherms models and
they are shown in Table 11.
Table 11. Adsorption isotherm models [Verma et al. (2006)].
Model
Equation
Langmuir
qe =
q m k L Ce
1 + k L Ce
1/n
Freundlich
qe = k F Ce
Dubinin –
Raduskevich
qe = qm exp(−𝐵𝑒 2 )
Temkin
qe = B1 ln(𝑘 𝑇 𝐶𝑒 )
68
3.4.2 Adsorption kinetics of metals
Adsorption kinetics of hexavalent chromium provide important data that can be used for
modeling of continuous flow packed bed adsorbers. The adsorption tests were performed in a
basket-typed reactor at room temperature. A know mass of activated carbon (1000 mg) was
added to 1L of solution with 30 ppm of Cr(VI) at pH 6. The reactor stirred at 400 rpm and the
pH value of the solution was only continuously recorded and the selected interval of time,
aliquots were taken for determining hexavalent and total chromium by the colorimetry method
coupled to UV-Vis spectroscopy and atomic absorption spectroscopy, respectively.
3.4.3.1 Empirical models
The adsorption kinetics of several metals have been predicted by using various proposed
empirical models (Table 12), such models can provide important information in agreement
with the theoretical basis on which they were developed.
Table 12. Empirical models of adsorption kinetics [Mittal et al. (2006)].
Kinetic model
Equation
First order
q t = q e − exp(𝑘1 𝑡)
Pseudo-first order
q t = q e [1 − exp(−k1 t)]
Second order
𝑞𝑡 =
𝑞𝑒
1 + 𝑞𝑒 𝑘2 𝑡
Pseudo- second order
𝑞𝑡 =
𝑘2 𝑞𝑒2 𝑡
1 + 𝑘2 𝑞𝑒 𝑡
69
3.4.3.2 Diffusive models
Just a few researchers have reported studies using diffusion models predicting the adsorption
kinetics of metals in lignocellulosic materials and activated carbon, some of these models are
shown in Table 13. The mass transfer phenomenon is vital in the adsorption process, as a
result, it cannot be omitted when analyzing the adsorption kinetics.
However, the kinetic processes are mostly controlled by the diffusion in aqueous phase or by
intraparticle diffusion, being this step the limiting rate; for this reason, the models are mainly
designed to describe these two diffusive processes [Kannan et al. (2001)].
Table 13. Diffusive adsorption kinetic models [Qiu et al. (2009)].
Kinetic model
Equation
Diffusion in the liquid phase
Cooney
∂q̅
= k f So (C − Ci )
∂t
Boyd
ln (1 −
R1 =
qt
) = −𝑅1 𝑡
qe
3D1e
𝑟0 ∆𝑟0 𝑘′
Intraparticule diffusion
𝑞
𝐷
6
Cooney
ln (1 − 𝑞 ) = − 𝑅2𝑠 𝑡 + ln (𝑝)
Weber & Morris
qt = k int t1/2
Dumwald & Wagner
log (1 − (
0
70
qt 2
𝐾
) )=−
𝑡
qe
2.303
3.4.4 Desorption studies
The possibility of adsorbent regeneration (desorption) and the metal recovery are the mainly
studied topics based on the general assumption that the adsorbent promotes an economic
adsorption treatment. It has been reported the desorption of Cr(VI) using as eluent alkaline
solutions such as: NaOH, Na2CO3, or NaHCO3 [Isa et al. (2008)].
The desorption test of hexavalent chromium from activated carbon were performed using the
following procedure: After the adsorption test, activated carbon was separated from the
solution of hexavalent chromium, and it was placed in a new plastic tube with 25 mL of a
solution of 0.1N NaOH-NaCl, with the objective to have two exchangeable anions (OH- , Cl-)
besides to control the solution ionic strength.
Desorption test was allowed in stirring for a week to simulate the same conditions in that the
adsorption test was performed. After one week of contact, an aliquot was taken for analysis by
colorimetry coupled to ultraviolet-visible and, in this way, the concentration of desorbed Cr(VI)
from the adsorbent material was determined.
3.5 Residues disposal
The general disposal of residues generated during this investigation project will be according
with the integral manage of residues and normative aspects from Faculty of Chemical Sciences,
UANL.
71
MASTER IN SCIENCE WITH ORIENTATION IN SUSTAINABLE PROCESSES
CHAPTER 4
Results & Discussion
“Research is to see what everybody
else has seen, and to think what
nobody else has thought.”
Albert Szent-Gyorgy
4.1 Characterization of adsorbent
4.1.1 Acid and basic sites: Boehm Method
The determination of acid (Phenolic, lactone and carboxylic) and basic sites were estimated
according to the established in the methodology for the Boehm method, and the summary of
results are shown in Table 14.
Table 14. Acid and basic sites of the studied activated carbon (Boehm method).
Activated
carbon
Phenol
group
(mEq/g)
Lactone
group
(mEq/g)
Carboxylic
group
(mEq/g)
Total acid
sites
(mEq/g)
Total basic
sites
(mEq/g)
AC raw
AC Ox(10%)
AC Ox(20%)
AC Ox(40%)
AC raw FH
AC Ox(10%) FH
AC Ox(20%) FH
AC T800
AC T900
AC T800 FH
AC T900 FH
AC NT
AC NT FH
1.00
4.00
2.25
1.38
1.62
0.50
0.88
0.00
0.00
1.25
1.25
1.25
1.25
0.75
0.25
0.50
0.25
0.00
0.00
1.00
0.00
0.00
0.00
0.00
0.00
0.00
1.00
0.50
1.00
0.88
1.13
2.00
1.00
0.00
0.00
0.00
0.00
0.00
0.00
2.75
4.75
3.75
2.50
2.75
2.50
2.88
0.00
0.00
1.25
1.25
1.25
1.25
0.38
2.25
0.38
1.88
2.00
2.00
2.00
2.50
2.38
2.38
2.38
2.00
2.25
Abbreviation
AC raw
AC Ox ( )
AC T#
AC NT
FH
Description
Commercial activated carbon
Oxidized Activated carbon with % HNO3.
Activated carbon with thermal treatment at temperature #.
Activated carbon modified with NH4OH.
Forced hydrolysis
73
Analysis of the results obtained with Boehm method revealed a greatly increased in the total
acidity of the surface that resulted from the increase in surface acid functional groups such as
carbonyl, lactone, and phenol groups, according to found by Huang et al. (2009), the total acid
sites (TAS) of raw adsorbent increased up to 72% with the nitric acid oxidation, reflecting an
increase in phenolic groups, Nevskaia et al. (2003) reported a similar behavior for activated
carbon oxidized with 10% HNO3, however, the TAS decreased with the oxidation degree, in
addition was shown an increase in the amount of the total basic sites (TBS) according with the
increase of the oxidation of the activated carbon.
Forced hydrolysis was performed to the activated carbon with lesser oxidation degree due to
the AC Ox(40%) presented a decrease of 48% of TAS with respect to AC Ox(10%); Forced
hydrolysis of these adsorbent materials caused changes in the structure of the acid sites of the
both materials raw and oxidized, decreasing to 47% TAS of the oxidized adsorbents and
increasing 5 times the amount of TBS of the commercial activated carbon.
In concerning the thermal treatment, the objective was reached with the modification of the
surface of the adsorbent material, due to heat treatment causes the removal of an important
part of the oxygen surface groups or oxygen surface complexes (phenol, lactone and carboxylic
groups) from the activated carbon, resulting an increase of 6 times the amount of TBS
regarding commercial activated carbon, Belhachemi et al. (2011) reported similar results with
activated carbon with thermal treatment at 600oC.
74
Thermally treated samples have a basic character which is due to some kind of oxygencontaining groups and mainly to the electron rich oxygen-free sites located on the carbon basal
planes. Nonetheless, the thermal treatment developed to 900oC showed a decrease of 5% TBS
with respect to the modified carbon at 800oC, suggesting the volatilization of some oxygenated
groups which acted as basic sites, analogous results were found by Attia et al. (2006) with
activated carbon heated at 400oC and 600oC for 2h. The effect of heat treatment is summarized
as follows, the same way as the treatment temperature increased, the amount of oxygenated
surface functional groups decreased until their complete elimination. At the start, the decrease
of carboxylic groups primarily occurred and the ketone or quinone structure began to
disappear when the treatment temperature further increased, a similar behavior was reported
by Shin et al. (1997) with activated carbon modified at 600, 1100 oC and for an hour in inert
atmosphere.
On the other hand, the results of forced hydrolysis of the adsorbents thermally treated
developed an increase of 1.25 mEq/g of phenolic groups compared with the heat-treated
materials without hydrolysis, remaining constant the amount of TBS. The increased in the
surface acidity indicates that the conditions in which the hydrolysis of the thermal-treated
adsorbents were carried out permitted the formation of new structures containing oxygen
above the activated carbon surface.
75
The effect of nitrogen treatment of the raw activated carbon (AC NT) suggested the rupture of
lactone and carboxylic groups, remaining only phenolic groups in 1.25 mEq/g, also causing the
enhancement of alkalinity, the objective of this modification was reached achieving an increase
of 5 times the amount of TBS regarding commercial activated carbon, this increase of the total
concentration of basic sites was attributed to the decrease of oxygen-containing acid groups
and the formation of nitrogen-containing basic groups via acid sites reacting with the ammonia
solution, this effect was also found by some authors ( Yang et al. (2014); Cao et al. (2014)) in
research about the amination and the nitration of activated carbon. However, Salame et al.
(2001) reported that the increase in alkalinity was due to the introduction of nitrogen species
and changes in the chemistry of the activated carbon surface. Nonetheless, forced hydrolysis of
adsorbent with nitrogen treatment (AC NT FH), did not provide any effect on the amount of
total acid sites of nitrogenized activated carbon; hydrolysis caused only an increase of 12% in
total basic sites of this adsorbent material.
Boehm method was a technique that allowed to characterize the acid and basic sites of the
different adsorbent materials in base to selective neutralizations, nevertheless, this technique
did not allow to identify specific basic sites. The results obtained by this method shown the
fulfilment of the planted objectives of the different treatments applied to commercial activated
carbon; an increase in the TAS by an oxidation with HNO3; a complete removal of TAS on the
surface of activated carbon by a thermal treatment as well as an increase of TBS through a
modification of the commercial activated carbon with NH4OH.
76
4.1.2 Elemental analysis
Adsorbents were subjected to differential thermal analysis to determine the content of oxygen
and ash, also was coupled to an elemental analysis for a better characterization of the
adsorbent, the results obtained are shown in Table 15.
Table 15. Overview of elemental analysis of the studied adsorbents.
Activated
carbon
Elemental content (wt %)
C
H
O
N
S
Ash
AC Ox(water)
72.91
76.63
1.65
0.53
16.20
13.84
0.54
0.67
0.70
0.33
8.00
8.00
AC Ox(10%)
65.12
1.50
18.65
1.19
0.54
13.00
AC Ox(20%)
65.77
1.43
31.22
1.09
0.49
0.00
AC Ox(40%)
69.06
1.24
21.92
1.30
0.48
6.00
AC raw FH
57.91
2.50
29.14
4.60
0.85
5.00
AC Ox(10%) FH
69.22
1.08
20.95
1.13
0.62
7.00
AC Ox(20%) FH
1.13
0.41
21.05
20.10
1.15
0.72
0.55
0.25
9.00
AC T800
67.12
68.52
10.00
AC T900
78.60
0.24
15.29
0.69
0.18
5.00
AC T800 FH
61.97
2.76
26.76
2.34
1.17
5.00
AC T900 FH
66.01
2.44
22.41
3.09
1.05
5.00
AC NT
64.95
4.52
24.02
4.02
0.49
2.00
AC NT FH
67.60
3.77
22.80
4.70
1.13
0.00
Ac raw
Abbreviation
AC raw
AC Ox ( )
AC T#
AC NT
FH
Description
Oxidized Activated carbon with water or HNO3.
Forced hydrolysis
77
Elemental analysis of the adsorbents showed an increase in the content of nitrogen and in
the fixation of the acidic functional groups on the commercial activated carbon surface
during the oxidation with nitric acid, the changes in activated carbon surface suggested a
mechanism proposed by Wehzhong et.al. (2008) similar to the reaction involving the
oxidation of aromatic hydrocarbons shown in Figure 13. The first step, reaction (a),
suggested the formation of the dicarboxylic group on the aliphatic side of the molecule
specially if consist of more than one carbon atom.
In the reaction (b), occurs the oxidation involving a methylene group which would result in
the formation of a ketone. Nitrogen can be added to the carbon by a similar reaction as in
the nitration of benzene, this mechanism would involve the formation of the highly
reactive nitronium ion (NO2+), which will be ultimately the nitrated product as shown in
reaction (c). However, in the case of the oxidized carbons the nitrated product would
appear in small quantities due to the limited amount of the nitronium ion since its
formation is favored in the presence of concentrated sulphuric acid.
78
Figure 13. Mechanism of oxidation of aromatic hydrocarbons: (a) 9, 10 Dihydrophenanthrene,
(b) Diphenylmethane, (c) Benzene [Wehzhong et al. (2008)].
On the other hand, thermal treatment can remove impurities such as hydrogen, nitrogen and
oxygen as the temperature increases due to most of the acidic oxygen-containing functional
groups are unstable and are subsequently removed, which also was observed by Figueiredo et
al. (1999) and Moreno-Castilla et al. (2004). Elemental carbon content also increases with
increasing thermal treatment temperature, Zheng et al. (2002) reported that is well known that
thermal treatment of carbon at high temperature under an inert atmosphere brings a more or
less graphitization, also enhances the basicity and the electronic conductivity of the carbon
support.
79
Forced hydrolysis of the heat-treated activated carbons (AC T800 FH, AC T900 FH) caused an
increased in the amount of sulfur compared with the adsorbents without hydrolysis, this result
was obtained because of the acid solution of iron, pH of the solution was adjusted with
sulphuric acid, that was used to modified the two thermal activated carbons, also was observed
an lightly increased in the amount of hydrogen and oxygen which could be attributed as an
effect of the acid media of the forced hydrolysis modification.
The modification of the activated carbon with an ammonia solution allowed the introduction of
nitrogen-containing functional groups cause by the increase in the amount of elemental
nitrogen in the adsorbent compared with the commercial activated carbon, this is consistent
with the results obtained with Boehm method, also was modified the amount of amount of
hydrogen and oxygen as a result of the strong solution used to modified the adsorbent.
Szymanski et al. (2004) reported that the modification with ammonia solutions enhanced the
basic properties of carbons at the expense of their acid properties, this kind of modifications
allowed the incorporation of nitrogen mainly, but also they reported an increase in amount of
oxygen especially when a foreign material as iron is incorporated in the adsorbent. As a result
of the forced hydrolysis (AC NT FH), an increase in the amount of elemental sulfur was showed
which it was a similar behavior obtained with the others adsorbents after their modification
with forced hydrolysis.
80
4.1.3 Fourier Transform Infrared spectrometry (FT-IR)
The qualitative analysis of surface functional groups of the adsorbents was performed with
the help of the infrared spectra, Figure 14, the spectrum of each adsorbent was analyzed
individually, and the identified bands are shown in Table 16 and 17.
Figure 14. Representative FT-IR spectra of the untreated carbon and modified samples.
81
IR assignments of the possible functional groups on carbon surfaces (raw and modified)
were summarized in Table 16. Commercial activated carbon presented important
absorption bands at specific wave numbers related with functional groups such as quinone,
carboxylic acid and lactones based on the review of the main characteristic peaks of
activated carbons in infrared spectra performed by Wenzhong et al. (2008).
Table 16. IR Assignments of functional groups on carbon surface.
Activated carbon
Group or functionality
Assignment region (cm-1)
AC raw
Quinone
1580
Carboxylic acid
1178,1699,2924
Lactone
1178,1699
Carboxylic acid
1120,1700,2700
Phenol
2700
Carboxylic acid
1120,1700,2650
Phenol
2650
AC T800
C-O in ethers (stretching)
1050
AC T900
None
None
AC NT
Carboxylic acid
1177,1588,3338
N-H, C = N
1580
C-N
1177
AC Ox(10%)
AC Ox(20%)
Abbreviation
AC raw
AC Ox ( )
AC T#
AC NT
FH
Description
Forced hydrolysis
82
On the other hand, the spectra obtained from the both activated carbons oxidized with
nitric acid indicated absorption bands of C-OH (stretching) and –OH related with phenolic
groups, also was identified the carboxylic acid, Moreno et al. (2000) reported same groups
or functionalities for activated carbons oxidized with 14M HNO3, nonetheless, other
authors (Khelifi et al. (2010); Allwar et al. (2012)) observed the presence of quinone or
conjugated ketone structure and carbonyl groups which have similar absorption bands, this
allowed that the peaks of these structures overlapped with other functional groups.
Nevertheless Bautista et al. (1994) reported that the nitric oxidation introduced a small
amount of nitrogen and attributed this observation to the formation of nitro and nitrate
aromatic compounds which also have similar absorption peaks (1100-1300, 1560 cm-1) like
the acid groups, Chimgombe et al. (2006) corroborated this information finding the same
behavior with activated carbon oxidized with nitric acid for 9h.
With regard to the thermal treatment, the modification performed at 800 oC provoked the
removal of most of the functional groups from the activated carbon as a result just a peak
appeared at 1100 cm-1 which could be a C-O bond in ether or hydroxyl group as found by
Attia et al. (2006).
83
Nevertheless, when the temperature increased at 900oC, no characteristic peak is observed
which is consistent with the results obtain with the Boehm method due to the elimination
of all acid groups or volatile compounds from the activated carbon surface, this a typical
effect found in the studies performed by Aguilar et al. (2003), Bandosz et al. (2006) and
Ríos et al. (2007) those who reported the decomposition of oxygenated groups when the
activated carbon is submitted to heat treatment at high temperatures specially under
hydrogen atmosphere.
The analysis performed to the spectrum of the activated carbon modified with ammonia
solution (AC NT) showed characteristic bands at 1580 and 1177 cm -1, which could be
attributed to N-H or C=N and C-N bonds, respectively. In addition, the spectrum have the
three absorption bands (1177, 1580 and 3338 cm -1) that characterize the carboxylic acid
group, but also at the same wave number could have overlapping bands of O-H and N-H
stretching vibrations due to that the nitrogen treatment suggest the formation of
nitrogenized species on the activated carbon surface, a similar analysis was performed by
Mohammad et al. (2011) in the study of ammonia modification of activated carbon and by
Adebukola & Min (2013) in their studies about basic treatments of activated carbon for
enhanced CO2.
84
The main absorption bands obtained from the spectra of the adsorbent materials with
forced hydrolysis are shown in Table 17. The effect of the hydrolysis on the adsorbent did
not cause changes in the IR spectrum of the commercial activated carbon, this behavior
corresponds to the obtained data from the Boehm method (AC raw FH), which showed that
the total acid groups remained constant after the forced hydrolysis of the commercial
activated carbon , and the oxygenated functional groups were still staying on the activated
carbon surface, this could explain why the absorption bands of AC raw FH had not changes
compared to the untreated carbon (AC raw).
Table 17. IR Assignments of functional groups on carbon surfaces with forced hydrolysis.
Activated carbon
Group or functionality
Assignment region (cm-1)
AC raw FH
Quinone
1580
Carboxylic acid
1170,1700,2910
Lactone
1180,1700
Carboxylic acid
1120,1700,2700
Phenol
2700
Lactone
1120,1740
Carboxylic anhydride
1120,1740
Phenol
1200,3000
-C-OH (stretching)
1200
Phenol
1200,3000
-C-OH (stretching)
1200
Carboxylic acid
1200,1750,3000
Phenol
1200,300
C-N
1200
AC Ox(10%) FH
AC Ox(20%) FH
AC T800 FH
AC T900 FH
AC TN FH
85
Same results were observed with the hydrolysis of the activated carbon oxidized with nitric
acid (AC Ox(10%) FH) which have similar behavior like the forced hydrolysis of commercial
activated carbon, the spectrum of this adsorbent indicated the presence of carboxylic and
phenolic groups mainly. However, according with the potentiometric titrations, this
activated carbon suffered a decrease in acid groups but the predominant groups still being
phenolic group and in higher amount carboxylic, that are same groups that appeared in the
oxidized activated carbon without forced hydrolysis, obtaining as a result the same infrared
spectrum. In contrast, AC Ox(20%) FH showed absorption peaks at 1200 and 1740 cm-1,
that according with Wenzhong and co-workers, are specific absorption band for carboxylic
anhydride and lactone groups, the effect of the hydrolysis in this adsorbent provoked the
decrease in acid surface groups, remaining in greater proportion lactone and carboxylic
group, this resulted in a grander intensity of the absorption bands at the wave numbers
related with these functional groups.
Forced hydrolysis of both heat-treated activated carbons caused the formation of new acid
structures although they looked in the spectra with lower intensity. Both adsorbents
presented the same absorption peak at 1200 cm-1 which is related with the –C-OH
(stretching) bond and together with the peak that appeared at 3000 cm -1 could suggest the
presence of phenolic groups which is the functional group found with the Boehm method
through potentiometric titrations.
86
Even though both thermal-treated activated carbons have the same absorption peaks in
the spectra, the activated carbon treated at 900oC and modified with forced hydrolysis
presented peaks with lower intensity, the absorption bands decreased about 15% in
transmittance compared with the activated carbon treated at 800oC and also modified with
forced hydrolysis, this could indicate that the functional groups in the second adsorbent
(AC T800 FH) are stronger linked to the activated surface than the first one, resulting in
higher vibrations in bonds and also in a greater intensity in the corresponding absorption
peaks.
IR spectrum of the activated carbon treated with an ammonia hydroxide solution and
modified with forced hydrolysis presented the same peaks like the activated carbon with
nitrogen treatment without hydrolysis, however in the wave number range from 1200 to
1750 cm-1, the peaks had higher intensity than the adsorbent without hydrolysis.
Nonetheless, the spectrum of the first adsorbent (AC NT FH) showed a lower percentage of
transmittance respect to the second activated carbon (AC NT), the absorption bands
decreased about 20% transmittance, this could be explained due to the AC NT FH had an
increase in the total basic sites this indicate that new structures were formed and they
could obstruct the radiation absorption resulting in a spectrum with lesser intensity or
lower transmittance percentage.
87
4.1.4 Physical adsorption of Nitrogen
The modified activated carbons during this research project were characterized by physical
adsorption of Nitrogen (Table 18), to obtain the surface area by BET method and pore size
distribution by BJH method.
Table 18. Surface area and pore size distribution of the adsorbent materials.
BET analysis
Pore size distribution (BJH)
% Pore Volume
Surface
area
(m2/g)
Pore
volume
(cm3/g)
Micropores
Mesopores
AC raw
AC Ox(10%)
735.30
858.40
3.39
4.23
7.21
7.96
84.07
84.42
8.72
7.62
1.60
1.50
AC Ox(20%)
807.70
4.08
7.30
85.00
7.70
1.56
AC Ox(40%)
779.00
3.88
7.16
84.39
8.44
1.68
AC raw FH
746.61
3.85
6.66
84.26
8.32
1.73
AC Ox(10%) FH
752.30
3.81
7.06
83.58
7.70
1.58
AC Ox(20%) FH
807.00
3.94
8.35
85.66
7.58
1.46
AC T800
785.30
4.10
7.67
89.50
8.65
1.71
AC T900
869.10
4.19
7.72
84.21
8.07
1.56
AC T800 FH
730.30
3.72
7.92
84.32
7.76
1.58
AC T900 FH
745.40
3.08
7.59
84.25
8.16
1.55
AC NT
1365.0
11.40
4.81
86.45
8.70
2.91
AC NT FH
1280.1
10.50
4.79
86.43
8.78
2.89
Activated
carbon
Abbreviation
AC raw
AC Ox ( )
AC T#
AC NT
FH
Average pore
diameter
Macropores
(nm)
Description
Forced hydrolysis
88
The results of the physical adsorption of nitrogen of the studied activated carbons, showed that
oxidation with nitric acid of the adsorbent materials caused an increase in up to 17% of BET
surface area and 24% of pore volume regarding commercial activated carbon. Nonetheless, it is
suggested that as the degree of the oxidation increased (AC Ox(40%)), activated carbon walls
began to collapse being reflected in a decrease of 10% of BET area and 25% of pore volume,
this same effect was found by many authors (Aksoylu et al. (2001); Nevskaia et al. (2003);
Stravropolus et al. (2008); Pietrzak et al. (2009)) whom studied the effects about the oxidation
of activated carbons with nitric acid.
Nonetheless, Huang et al. (2009) suggested that the decreased in BET surface areas resulted
from a pore blockage in the micropores caused by the formation of humic substances or
surface oxides during the oxidation process. However, Khelefi et al. (2010) reported that the
considerable changes in the activated carbon surface was also could be due to by two possible
effects: i) the partial destruction of the micropores walls and ii) the restrictions of the
accessibility caused by the fixation of oxygen containing groups at the entrance of the pores.
Due to the drastic nature of the acid treatment which partially broke the pores texture
(observed in the decrease in BET area) of the activated carbon probably did not favor the
formation of ketone and ether groups resulting in a decrease of acid surface groups as was
observed in the Boehm method, which also was found by Moreno et al. (1995) in their studies
about the modification of the activated carbon surface.
89
On the other hand, both adsorbents (commercial activated carbon such as the activated carbon
oxidized with nitric acid) contain up to 85% of mesopores which was confirmed with the
adsorption-desorption isotherms of nitrogen type IV, typical for mesoporous materials.
The results of the force hydrolysis of the commercial activated carbon (AC raw HF) showed an
increase of 1.5% on the BET surface area and 14% in pore volume respect to the raw adsorbent,
this indicated that the hydrolysis provoked the formation of a higher amount of pores in the
activated carbon how was expected due to the experimental conditions of the forced
hydrolysis which acted like an acid treatment cause by the sulfuric acid media and heating at
70oC.
Furthermore, the effect of the forced hydrolysis in both oxidized adsorbents resulted in a lightly
decrease (about 11%) of the BET surface area and pore volume, compared with the adsorbents
without hydrolysis, cause by the strong conditions in which the hydrolysis was carried out (acid
solution and heating) that could have had a similar effect like the oxidation with nitric acid,
causing the breaking down of the walls of the pores modifying the surface area and pore
volume, however, after the hydrolysis the oxidized activated carbons still continue being mainly
mesoporous.
90
The analysis of the heat-treated activated carbons showed a slight increase in the BET surface
area about 7% and important increase of 20% in pore volume compared with commercial
activated carbon, this behavior resulted from the elimination of oxygenated groups or the
elimination of volatile matter of the activated carbon surface causing the generation of new
pores in the adsorbent increasing BET surface area; Chimgombe et al. (2005) reported that
thermal treatment removed some of the humic acid lodged in micropores and eliminates some
oxygen-containing functional groups that are not stable at high temperatures, increasing
surface area. Other authors (Rangel et al. (2005); Yoo et al. (2005); Ruiz et al. (2007)) obtained
a physical improve, mainly in BET area and micro and mesopore volumes, on the adsorbent
properties as a result of the heat treatment of activated carbon at high temperatures.
Furthermore, forced hydrolysis of the heat-treated adsorbents (AC T800 FH, AC T900 FH)
showed a decreased in BET surface area, compared with the thermal-treated activated carbons
without hydrolysis, as a result of the acid solution used in the forced hydrolysis modification
which could be collapsed the walls of the activated carbon diminishing the surface area and
obtaining a slight increase in pore volume, also this behavior could suggest that the iron
particles were distributed on the activated carbon surface diminishing the BET area, this effect
have been reported by many authors (Ang et al. (2000); Park et al. (2003); Bulushev et al.
(2004)) who modified the surface of catalyst supports with a foreign material mainly metals
such as iron, copper, gold, silver among others.
91
The incorporation of nitrogen on the commercial activated carbon (AC NT) caused an important
effect on the adsorbent surface increasing the BET area about 85% and 3 times the pore
volume in comparison with the raw activated carbon; however, forced hydrolysis of this
nitrogen-containing activated carbon (AC NT FH) showed a slight decrease in the surface area
around 6% and 8% in pore volume compared with AC NT, that is a similar behavior obtained
from the others activated carbons with anchored iron particles. Both adsorbents AC NT and AC
NT FH showed a decrease about 50% in their densities as a result of the changes in the surface
area and pore volume caused by the strong nitrogen treatment.
Biniak et al. (1997) modified the activated carbon surface by an ammonia treatment at high
temperature, and they observed an increase of microporosity that was in agreement with data
for other ammonia treated carbons; the same author in 1999, reported that the amination
process alters the surface area (BET) only slightly, but strongly influences the surface chemical
structure of the activated carbon. Same results were obtained by Mangun et al. (2001) who
reported that the ammonia treatment allowed the increasing of pore size and surface area,
also Lei et al. (2002) observed amination had little effect on micropore volume and BET surface
area, then had no noticeable effects on pore size distribution.
92
4.2 Batch adsorption-desorption test
4.2.1 Cr(VI) adsorption test
The adsorption test of hexavalent chromium were performed up to the equilibrium was
reached, using the different adsorbent materials generated during this researcher project,
isotherms were classified according to the modification performed to the activated carbons
also the parameters of the adsorption isotherm model which were used to adjust the data are
shown.
4.2.1.1 Adsorption isotherms: Commercial activated carbon
In Figure 15, are shown the adsorption isotherms of the commercial activated carbon as well as
its modification with forced hydrolysis; data fitting was performed to different models of
adsorption isotherms reporting the best fit in Table 19.
Table 19. Parameter of the fitting to Freundlich for adsorption isotherms.
Correlation
K
qmax
factor
Specie
1/n
(mg/g)*(mg/L)-1/n
(mg/g)
R2
AC raw
AC FH
0.8745
5.3350
0.6490
0.3676
0.9959
0.9856
45.6
49.5
From the analysis performed to adsorption isotherms, a same tendency of the isotherm of the
activated carbon with forced hydrolysis was observed in comparison with the isotherm of the
commercial activated carbon.
93
The hydrolysis of the commercial activated carbon presented as important effect a greater
adsorption capacity (50%) by the adsorbate to concentrations lesser than 150 mg/L and about
25% at upper concentrations compared with the raw material. Equilibrium data of the batch
adsorption tests of both adsorbents were fitted to different adsorption isotherms models,
however, the fitting which better described the behavior of the experimental data was the
Freundlich model.
60
q (mg Cr(VI)/g CA)
50
40
30
20
10
0
00
50
100
150
200
250
300
350
400
450
Ce (mg/L)
Figure 15. Adsorption isotherm of Cr(VI) on activated carbon (2 mg CA/mL, pH = 6 and
temperature of 25oC); Freundlich fit (continuous line ):  AC raw,  AC raw HF.
94
4.2.1.2 Adsorption isotherms: Oxidized activated carbons with HNO3
Adsorption equilibrium data of Cr(VI) of the adsorbents oxidized with nitric acid as well as its
modification with forced hydrolysis are shown in Figure 16. Analysis of the adsorption tests
indicated that the activated carbons oxidized with HNO3 presented a lesser adsorption capacity
in all the range of concentration of the adsorbate studied in comparison with the commercial
adsorbent.
50
45
40
q (mg Cr(VI)/g CA)
35
30
25
20
15
10
5
0
00
50
100
150
200
250
300
350
400
450
Ce (mg/L)
Figure 16. Adsorption isotherms of Cr(VI) on activated carbon (2 mg CA/mL, pH = 6 and
temperature of 25oC); Freundlich fit (continuous line):  AC raw,  AC Ox(10%),
 AC Ox(20%),  AC Ox(10%) FH,  AC Ox(20%) FH.
95
Nevertheless, the oxidized and modified adsorbents with forced hydrolysis showed an
increase of 25% on adsorption capacity of the commercial activated carbon only at
concentrations of Cr(VI) < 80 mg/L, and above this value of concentrations the equilibrium
data showed a similar behavior to the activated carbons only oxidized with nitric acid,
which presented a lower Cr(VI) adsorption capacity compared to the raw adsorbent.
The equilibrium data obtained were fitted to different adsorption isotherm models,
obtaining the best fit with the Freundlich model. The summary of the parameters of the
model is shown in Table 20, where it was observed that the commercial activated carbon
presented the greater adsorption capacity (45.6 mg/g) compared with the others activated
carbons oxidized with nitric acid as well as the modification of these carbons with forced
hydrolysis.
Table 20. Parameter of the fitting to Freundlich for adsorption isotherms (Oxidation HNO3).
Correlation
Activated
K
qmax
1/n
-1/n
factor
carbon
(mg/g)*(mg/L)
(mg/g)
R2
AC raw
AC Ox(10%)
AC Ox(20%)
AC Ox(10%) FH
AC Ox(20%) FH
0.927
1.207
1.405
5.603
5.258
0.634
0.507
0.525
0.2662
0.2284
96
0.9965
0.9798
0.9808
0.9695
0.9687
45.60
27.82
32.99
30.23
22.99
4.2.1.3 Adsorption isotherms: Activated carbons with thermal treatment
Adsorption equilibrium data of Cr(VI) of the activated carbons with thermal treatment, Figure
17, followed a similar behavior to the commercial activated carbon; on the other hand, forced
hydrolysis of these adsorbents caused an increase of 26% on maximum adsorption capacity of
raw adsorbent and a higher affinity to concentrations of Cr(VI) lesser than 200 mg/L.
70
60
q (mg Cr(VI)/g CA)
50
40
30
20
10
0
00
50
100
150
200
250
300
350
400
450
Ce (mg/L)
Figure 17. Adsorption isotherms of Cr(VI) on activated carbon (2 mg CA/mL, pH = 6,and
temperature of 25oC); Freundlich fit (continuous line ):  AC raw,  AC T800,
 AC T900,  AC T800 FH,  AC T900 FH.
97
Adjusting of equilibrium data obtained for the different adsorbents with thermal treatment
was performed, resulting Freundlich model the most adequate for data and summarizing the
information obtained in Table 21, in which it was observed that the activated carbon with
thermal treatment at 900oC and modified with iron particles presented the greater adsorption
capacity (59.82 mg/g) in comparison with the rest of the adsorbents.
Table 21. Parameter of the fitting to Freundlich for adsorption isotherms (Thermal treatment).
Correlation
Activated
K
qmax
1/n
-1/n
factor
carbon
(mg/g)*(mg/L)
(mg/g)
R2
AC raw
AC T800
0.927
1.776
0.634
0.537
0.9965
0.9891
45.60
43.33
AC T900
1.518
0.5867
0.9884
48.85
AC T800 FH
10.970
0.274
0.9819
57.82
AC T900 FH
18.200
0.202
0.9839
59.82
The increase on the adsorption capacity of the adsorbents with thermal treatment in
comparison with the commercial activated carbon was attributed to the removal of the
electrostatic repulsion generated by acid sites of the surface of the adsorbents surface, in
addition of the increase of basic sites available for adsorption of Cr(VI) anions; moreover, the
incorporation of iron species onto activated carbons with thermal treatment acted as anchor
sites of Cr(VI) causing an increase of 50% on the adsorption capacity of the commercial
activated carbon at concentration of Cr(VI) lesser than 200 mg/L.
98
4.2.1.4 Adsorption isotherms: Activated carbon with nitrogen treatment
The effect of the modification of the activated carbon surface with ammonium hydroxide
resulted in an increase on the adsorption capacity of the commercial activated carbon by 7% in
the studied concentration range (Figure 18), the changes on the adsorption capacity were
attributed to increase of basic sites generated by the nitrogen treatment. In addition,
hydrolysis of the adsorbent with nitrogen treatment produced a higher affinity for the
adsorbate up to concentrations of Cr(VI) lower than 250 mg/L.
60
q (mg Cr(VI)/g CA)
50
40
30
20
10
0
00
50
100
150
200
250
300
350
400
450
Ce (mg/L)
Figure 18. Adsorption isotherms of Cr(VI) on activated carbon (2 mg CA/ml, pH = 6, temperature of
25oC); Freundlich fit (continuous line ):  AC raw,  AC NT,  AC NT FH.
99
The analysis of the adsorption equilibrium data of Cr(VI) shown that the incorporation of
iron species on the nitrogenized activated surface increased affinity for the adsorbent to
low concentrations about 60% on the adsorption capacity of the commercial activated
carbon. Isotherms were fitted to Freundlich model, parameter values of the model are
reported in Table 22, where even though nitrogenized activated carbon (AC NT) presented
the greatest adsorption capacity, its modification with forced hydrolysis (AC NT FH) was
chosen since it presented a higher affinity for the adsorbent at low concentrations.
Table 22. Parameter of the fitting to Freundlich for adsorption isotherms (Nitrogen treatment).
Activated
K
Correlation factor
qmax
1/n
-1/n
2
carbon
(mg/g)*(mg/L)
R
(mg/g)
AC raw
AC NT
0.927
1.334
0.634
0.596
0.9965
0.9939
45.60
48.51
AC NT FH
22.500
0.125
0.9850
48.41
4.2.2 Cr(VI) reduction analysis
Reduction analysis of Cr(VI) during the adsorption process was an important factor that
helped to elucidate how interacted the species of chromium with the functional groups
present on commercial activated carbon as well as its diverse performed modifications.
100
4.2.2.1 Adsorption isotherms: Commercial activated carbon
In Figure 19, adsorption isotherms of total chromium and Cr(VI) are shown for the commercial
activated carbon (AC raw) as well as for its modification with forced hydrolysis (AC raw FH); in
the pair of isotherms for each adsorbent was observed a similar behavior of the equilibrium
data for the Cr(VI) as for total chromium indicating that the conditions in which took place the
adsorption process achieved a reduction about 2% from Cr(VI) to Cr(III) as a result of the
activated carbon surface charge and the low available of protons in solution.
50
45
qe (mg Cr / g CA)
40
35
30
25
20
15
10
5
0
0
50
100
150
200
250
300
350
400
450
Ce (mg/L)
Figure 19. Adsorption isotherms of Cr(VI) on activated carbon (2 mg CA/mL, pH = 6 and T = 25oC):
 AC raw Cr(VI),  AC raw total Cr,  AC raw FH Cr(VI),  AC raw FH total Cr.
101
4.2.2.2 Adsorption isotherms: Oxidized activated carbons with HNO3
In the analysis of isotherms of Total and hexavalent chromium for the activated carbons
treated by a oxidation with nitric acid (Figure 20), presented around 33% of reduction of Cr(VI)
to Cr(III) at concentrations greater than 100 mg/L, obtaining a high percentage of reduction in
comparison with 2% achieved by commercial activated carbon. On the other hand, no
reduction was shown at low concentrations due to basic surface groups allow the hexavalent
chromium adsorption and there was not enough protons to the electron donors to provoke a
Cr(VI) reduction.
50
45
qe (mg Cr / g CA)
40
35
30
25
20
15
10
5
0
0
50
100
150
200
250
300
350
400
450
Ce (mg/L)
 AC raw Cr(VI),  AC raw total Cr,  AC Ox(10%) Cr(VI),  AC Ox(10%) total Cr,
▲ AC Ox(20%) Cr(VI), ▲ AC Ox(20%) total Cr.
102
On the another hand, in the analysis of the isotherms of the adsorbents oxidized with nitric
acid and modified with forced hydrolysis was obtained a 50% of reduction of Cr(VI) to Cr(III)
at concentrations greater than 100 mg/L, this effect was attributed mainly to the density of
acid sites that acted as electron donors causing the Cr(VI) reduction, standing out the
commercial activated carbon as the best adsorbent of hexavalent chromium anionic
species.
60
qe (mg Cr/ g CA)
50
40
30
20
10
0
0
50
100
150
200
250
300
350
400
450
Ce (mg/L)
 AC raw FH Cr(VI),  AC raw FH total Cr,  AC Ox(10%) FH Cr(VI),  AC Ox(10%) FH
total Cr, ▲ AC Ox(20%) FH Cr(VI), ▲ AC Ox(20%) FH total Cr.
103
4.2.2.3 Adsorption isotherms: Activated carbons with thermal treatment
Regarding thermal treatment, isotherms of total chromium and Cr(VI) for the obtained
adsorbent materials, Figure 22, showed a similar tendency to the behavior obtained from the
activated carbons oxidized with nitric acid, which involved a 35% of Cr(VI) reduction at
concentrations above 100 mg/L , nonetheless presented a higher adsorption capacity with
regard to these adsorbent materials as a result of the increase in the quantity of basic surface
groups by the thermal treatment.
80
qe ( mg Cr/ g CA)
70
60
50
40
30
20
10
0
0
50
100
150
200
250
300
350
400
450
Ce (mg/L)
Temperature of 25oC):  AC raw Cr(VI),  AC raw total Cr,  AC T800 Cr(VI),
 AC T800 total Cr, ▲ AC T900 Cr(VI), ▲ AC T900 total Cr.
104
Forced hydrolysis of the adsorbents treated thermally, Figure 23, caused that iron species
anchored on the activated carbon surface showed 20% lower reduction of Cr(VI) to Cr(III)
compared with the materials without hydrolysis, this effect was attributed to the interaction of
electron donors with iron species inhibiting the Cr(VI) reduction during the adsorption process,
however, it was observed that was achieved only 1% of reduction to concentrations lesser than
100 mg/L and a maximum of 10% at higher concentrations for these adsorbents allowing
anionic adsorption at low concentrations.
80
70
qe ( mg Cr/ g CA)
60
50
40
30
20
10
0
0
50
100
150
200
250
300
350
400
450
Ce (mg/L)
Temperature of 25oC):  AC raw Cr(VI),  AC raw total Cr,  AC T800 FH Cr(VI),
 AC T800 FH total Cr, ▲ AC T900 FH Cr(VI), ▲ AC T900 FH total Cr.
105
4.2.2.4 Adsorption isotherms: Activated carbon with nitrogen treatment
Isotherm of the nitrogen-containing activated carbon (AC NT) showed a hexavalent chromium
reduction about 30% between 100-200 mg/L compared with the commercial activated carbon
(Figure 24). However, at upper concentration values (> 200 mg/L) the quantity of trivalent
chromium increased about 50%, this behavior suggested that some electron donor groups from
the activated carbon surface reduced the anionic chromium species. In another hand, forced
hydrolysis of activated carbon (AC NT FH), did not show reduction at concentrations below 150
mg/L, nonetheless above this concentration a 20% of reduction of Cr(VI) was obtained, this
behavior could explain that the hydrolysis provoked changes in the chemistry surface of the
adsorbent and iron particles allowed the anionic adsorption of Cr(VI).
60
qe ( mg Cr/ g CA)
50
40
30
20
10
0
0
50
100
150
200
250
300
350
400
450
Ce (mg/L)
Temperature of 25oC):  AC raw Cr(VI),  AC raw total Cr,  AC NT Cr(VI),
 AC NT total Cr, ▲ AC NT FH Cr(VI), ▲ AC NT FH total Cr.
106
4.2.2.5 General discussion: Hexavalent chromium adsorption
Commercial activated carbon showed a high adsorption capacity compared with some
adsorbents that have been studied, Table 23, most of the adsorption studies were carry out in
acid media because of the nature of the adsorbent that allowed the adsorption of hexavalent
chromium species but also provoked up to 98% of reduction to trivalent chromium.
The obtained results showed that the commercial activated carbon (AC raw) had a great
adsorption capacity (45.60 mg/L) at room temperature and pH = 6. In addition, forced
hydrolysis of the raw adsorbent (AC raw FH) caused an improvement about 46% in the
adsorption capacity (Figure 15) especially at low concentration of hexavalent chromium (lesser
than 200 mg/L), also the forced hydrolysis provoked a slightly increase about 5% in the
maximum adsorption capacity (49.50 mg/L) of the commercial adsorbent.
Table 23. Cr(VI) adsorption capacity for different adsorbent materials.
mg Cr6+/g
pH
Hazelnut shell
Activated carbon, FS-100
17.70
69.30
2.0
3.0
Adsorption
system
Batch
Batch
Activated carbon, fabric cloth
22.29
2.0
Batch
Mohan et al. (2005)
Terminalia arjuna nuts
28.40
1.0
Batch
Mohanty et al. (2005)
Bituminous coal
7.00
5.0 - 8.0
Batch
Natalie et al. (2007)
AC raw
45.60
6.0
Batch
This study (2014)
AC raw FH
49.50
6.0
Batch
This study (2014)
Raw material
107
Reference
Cimino et al. (2000)
Hu et al. (2003)
On the another hand, reported researches about the hexavalent chromium adsorption did not
involve the study of the adsorption mechanism, just some authors reported that during the
adsorption of chromium species in acid media or low pH values, a higher quantity of hexavalent
chromium was reduced to its trivalent form. However, the analysis of total chromium
performed to the activated carbons at pH = 6 showed only a 2% of reduction by the commercial
activated carbon because of the pH conditions in which the adsorption process was carried out
there were not an enough quantity of protons to interact with electron donors provoking
reduction to Cr(III); On the other hand, AC raw FH presented a reduction of up to 12% at
concentrations greater than 150 mg/L as a result of the acid surface groups incorporated in the
activated carbon surface during the hydrolysis step and at low concentration iron particles
working as an adsorption site of anionic chromium species decreasing the reduction rate.
Analysis performed to the oxidized activated carbon with nitric acid allowed to identify the pair
of the best adsorbents that resulted from the modification (AC Ox(10%), AC Ox(10%) FH) which
had a higher adsorption capacity than other modified adsorbents, Table 24. However, most of
the adsorption test performed by acid-treated adsorbents were carry out in acid media causing
an important amount of reduced hexavalent chromium (greater than 90 %). On the contrary,
adsorption test at pH = 6 allowed to decrease the reduction rate due to AC Ox(10%) showed
only a 33% of reduction at concentrations greater than 100 mg/L.
108
In addition, AC Ox(10%) FH increased the adsorption capacity about 25% at concentration
lower than 100 mg/L without reduction to trivalent chromium, but above this concentration
value about 50% of reduction was shown.
Table 24. Cr(VI) adsorption capacity for activated carbons oxidized with minerals acids.
Raw material
Commercial
Commercial
Commercial
Commercial
Commercial
AC Ox (10%)
AC Ox (10%) FH
Technique used
Oxidation using HNO3
Oxidation using sulfuric and
nitric acid
Oxidation using HNO3
Oxidation using HNO3 and
heat treatment
Oxidation using sulfuric and
nitric acid
Oxidation using nitric acid
Oxidation using nitric acid
and forced hydrolysis
pH
mg Cr6+/g
-
10.40
Babel et al. (2004)
-
13.30
Zhao et al. (2005)
2.0
13.70
Liu et al. (2007)
2.0
10.93
Kumar et al. (2009)
4.0
16.10
Huang et al. (2009)
6.0
27.82
This study (2014)
6.0
30.23
This study (2014)
Reference
Thermal treatment did not cause an important effect on the adsorption capacity of the
commercial activated carbon, both heat-treated activated carbons (AC T800, T900) showed a
very similar adsorption isotherms behavior compared with the raw adsorbent, there are few
studies about the adsorption of hexavalent chromium by adsorbents modified thermally, most
of them were coupled to another treatment such a oxidation using mineral acids (Table 25).
109
Besides, forced hydrolysis of the heat-treated activated carbons (AC T800 FH, AC T900 FH)
caused an important effect on the adsorption capacity of the adsorbents increasing 26% on
maximum adsorption capacity of raw adsorbent and a higher affinity at concentrations of Cr(VI)
lower than 200 mg/L. Even though AC T900 FH showed the greatest adsorption capacity, AC
T800 FH was chosen as a better adsorbent because of the increase of 100 oC on temperature
only generated an increased about 15% on the adsorption capacity at concentration lesser that
150 mg/L, and above this concentration both adsorbents had almost the same value in the
adsorption isotherms reaching similar maximum adsorption capacities, therefore is not justify
the increase in the consumption of energy with the improvement in the adsorption capacity.
Table 25. Cr(VI) adsorption capacity for activated carbons modified by thermal treatment.
Technique used
pH
mg Cr6+/g
Commercial
Oxidation using mineral acid
and thermal treatment
-
13.30
Zhao et al. (2005)
Commercial
Oxidation using HNO3 and
heat treatment
2.0
10.93
Kumar et al.(2009)
AC T800
Thermal treatment at 800oC
6.0
43.33
This study (2014)
6.0
57.48
This study (2014)
Raw material
Reference
o
AC T800 FH
Thermal treatment at 800 C
110
On the another hand, the adsorbents with thermal treatment presented about 35% of Cr(VI)
reduction at concentrations greater than 100 mg/L, nonetheless heat-treated activated carbons
modified with forced hydrolysis showed 20% lower reduction of Cr(VI) to Cr(III) compared with
the materials without hydrolysis as a result of the iron particles anchored on the activated
surface which allowed an anionic adsorption of chromium species diminishing the quantity of
reduction to trivalent chromium.
The modification of commercial activated carbon with ammonium solutions (AC NT) resulted in
an increase on the adsorption capacity about 7% compared with the raw adsorbent, however,
hydrolysis of the activated carbon containing nitrogen produced a higher affinity (26%) for
Cr(VI) up to concentrations lesser than 250 mg/L. The obtained adsorption capacities had an
average value compared with that obtained with other adsorbents, Table 26, as a result of the
method used to modify the activated carbon surface.
Table 26. Cr(VI) adsorption capacity for activated carbons modified by ammonia treatment.
Raw material
ACF-307
Coconut Pitch
Activated
carbon
AC NT
AC NT FH
Technique used
pH
mg Cr6+/g Reference
Oxidation using ammonium
persulphate
Amine modification
-
19.00
Aggarwal et al. (1999)
3.0
123.40
Unnithan et al. (2004)
Amine-crosslinked copolymer
5.0
102.88
Sun et al. (2013)
Oxidation using Ammonia solution
Oxidation using Ammonia solution
6.0
48.51
This study (2014)
6.0
45.41
This study (2014)
111
In addition, many of the adsorption studies performed with activated carbon modified with an
ammonia source did not analyze neither the reduction of Cr(VI) to Cr(III) nor the effect of the
nitrogen functional groups on the adsorption capacity. The results of the total chromium
isotherms showed that AC NT surface caused up to 50% of reduction of hexavalent chromium
at concentration greater than 100 mg/L; On the other hand, forced hydrolysis of the activated
carbon (AC NT FH) improved the adsorbent surface resulting in a 20% of Cr(VI) reduction at
concentration greater than 150 mg/L.
4.2.3 Cr(VI) desorption test
Desorption test were carried out from different adsorbents studied using a solution of NaOHNaCl 0.1 N, a desorption with this eluent suggested the way in which species of Cr(VI) and
activated carbon interacted indicating anionic adsorption, the obtained results are shown in
Figure 25. Adsorbents that shown the best results were commercial activated carbon with
forced hydrolysis (AC raw HF), activated carbon with thermal treatment 800oC and it forced
hydrolysis (AC T800 FH) as well as activated carbon with thermal treatment at 900 oC and also it
forced hydrolysis (AC T900 FH), resulting the activated carbon modified with thermal treatment
and forced hydrolysis as the best adsorbents in both adsorption and desorption. Some
researchers have reported an excellent desorption rate using acid solutions such as HNO3, HCl
or EDTA (Gupta et al. (2010); Kumar et al. (2011)), reaching up to 75% of the adsorbed
chromium.
112
However, desorption with acid solutions indicated cation exchange, therefore trivalent
chromium was the specie of chromium that took place in the adsorption process. On the
contrary, anion exchange was reported by Selvaraj et al. (2003) who achieved 82% of Cr(VI)
desorption using 0.014 M NaOH; and Deng et al. (2004) reported a 96% of hexavalent
chromium desorption using 0.5 M NaOH. The studied activated carbon achieved different
desorption capacities which is related with the adsorption process because of nitrogen and acid
nitric oxidation treatment caused a high reduction of hexavalent chromium while commercial
and heat-treated activated carbon showed a short reduction at low concentration; In addition,
forced hydrolysis improved the anionic adsorption of the adsorbent material, therefore the
best selected adsorbents (AC T800 FH, AC T900 FH) showed about 96% of Cr(VI) desorption
using 0.1 N NaOH-NaCl indicating also anion exchange during the desorption process.
16
14
q (mg/g)
12
10
8
6
4
2
0
Figure 25. Histogram of the adsorption-desorption analysis of the different adsorbent materials:
 Adsorption capacity (mg/g),  Desorption capacity (mg/g); Initial concentration: 30
mg/L Cr (VI), pH = 6.
113
4.2.4 Cr(VI) adsorption kinetics
The analysis of the adsorption kinetics of the selected adsorbents (Figure 26) showed a faster
adsorption of hexavalent chromium achieved by the heat-treated activated carbon with forced
hydrolysis (AC T800 FH) diminishing the equilibrium time (1031 min) about 30% compared with
the activated carbon without hydrolysis AC T800 (1300 min) and about 55% in comparison with
the commercial activated carbon (1600 min) indicating that thermal treatment coupled the iron
particles adsorption sites helped to the adsorbent surface to attach hexavalent chromium ions
improving both capacity as adsorption rate of the activated carbons.
1,0
Normalized concentration
0,9
0,8
0,7
0,6
0,5
0,4
0
500
1000
1500
2000
2500
3000
Time (min)
Figure 26. Adsorption kinetics analysis of the different selected adsorbent materials:
 AC raw, ▲ AC T800,  AC T800 FH. Initial concentration: 30 mg/L Cr (VI), pH = 6.
114
Adsorption kinetic data (AC T800, AC T800 FH) were pretty adjusted to a Pseudo-second order
model (Eq. 4.1), the main parameter were obtained and they are shown in Table 27.
𝑘2 𝑞𝑒2 𝑡
𝑞𝑡 =
1 + 𝑘2 𝑞𝑒 𝑡
(Eq. 4.1)
In the contrary, adsorption kinetic data of the raw activated carbon could not be adjusted
because its kinetic curve showed two adsorption steps, indicating the simultaneous adsorptionreduction mechanism of hexavalent chromium.
Table 27. Adsorption kinetic parameters.
Adsorbent
K2
qe
R2
AC T800
0.00144 12.42
0.99986
AC T800 FH
0.00162 15.08
0.99975
Ho et al. (1999) reported an extensive study about the pseudo-second order model for sorption
processes and they concluded that in their 12 different studied systems, chemical reaction
seems significant in the rate controlling step and the pseudo-second order model provides the
best correlation of the experimental data. Khezami et al. (2005) reported that pseudo-second
order model well mimics the adsorption kinetic data in the removal of chromium (VI) by
activated carbon. In addition, AL-Othman et al. (2012) reported the kinetics of Cr(VI) followed
both pseudo-first order and pseudo-second order rate expressions in the removal of
hexavalent chromium by activated carbon prepared from peanut shells.
115
MASTER OF SCIENCE WITH ORIENTATION IN SUSTAINABLE PROCESSES
CHAPTER 5
Conclusions
“A scientist in his laboratory is not a
mere technician: he is also a child
confronting natural phenomena
that impress him as though they
were fairy tales.”
Marie Curie
CONCLUSIONS
The oxidation treatment with nitric acid caused the increase in quantity of surface acid
groups, mainly fenolic and carboxylic groups according with Boehm titrations and the
obtained infrared spectra. Also the oxidized activated carbons showed an slightly
increment in the BET surface area about 16%; however, an increase in the oxidation degree
caused that activated carbon walls collapsed and, therefore, a 10% surface area and 25%
pore volume were shown.
Thermal treatment removed all the surface acid groups from the activated carbon surface
increasing the quantity of basic groups as a result of the decomposition of oxygenated
groups or volatile compounds causing the generation of new pores in the adsorbent that
slightly increase the BET surface area about 7% and 20% in pore volume compared with
commercial activated carbon.
Nitrogen treatment increased the quantity of nitrogen groups on activated carbon
according with the obtained results from elemental analysis and infrared spectra; In
addition, Boehm titrations showed that nitrogen containing activated carbons presented
an increment in the quantity of surface basic groups. On the other hand, nitrogen
treatment caused an important effects increasing the BET surface area about 85% and 3
times the pore volume.
117
Forced hydrolysis of the commercial and modified activated carbon did not show
important changes on surface chemistry of activated carbon, but decrease both BET
surface area and pore volume about 15 % as a result of pore blocking by anchored iron
particles. Iron analysis showed that activated carbon with higher quantity of acid functional
groups had the greater quantity (0.15%) of anchored iron particles and the adsorbents
without acid groups and high content of surface basic groups had the lower quantity
(0.01 %) of iron over adsorbent surface.
Commercial and modified activated carbon are able to sequester metal ions such as
hexavalent chromium from aqueous solution; however, oxidized and nitrogen containing
activated carbons caused about 40-50% of reduction of Cr(VI) to Cr(III), resulting as better
adsorbents those which were treated thermally.
On the other hand, the anchoring of iron particles improved the hexavalent chromium
adsorption of the activated carbons at concentration lower than 150 mg/L. All equilibrium
data of the different studied adsorbents were well-fitted to Freundlich model.
Even though, oxidized activated carbon had the higher iron quantity, heat-treated
adsorbents and modified by forced hydrolysis showed the best adsorption capacity (57
mg/g) of hexavalent chromium from aqueous solutions.
118
Hexavalent chromium desorption test showed that commercial and oxidized activated
carbons could desorb about
80% of hexavalent chromium, and nitrogen containing
activated carbon just could desorb about 30%; these low desorption percentages indicated
that the chromium removal was through a reduction coupled adsorption mechanism.
In addition, heat-treated activated carbons presented the higher desorption (98%) of
hexavalent chromium indicating that the main adsorption mechanism was carried out by
anion adsorption.
The best adsorbents resulted the heat-treated activated carbons and modified with forced
hydrolysis (T800 FH, T900 FH) who achieved an adsorption capacity of 57 mg/g and 98% of
Cr(VI) desorption.
119
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AUTOBIOGRAPHIC SUMMARY
Jonathan Valentín Reyes
Candidate for the Master Degree of
Sciences with Orientation in Sustainable Processes
Dissertation title: INFLUENCE OF FUNCTIONAL GROUPS OF ACTIVATED CARBON IN THE
ANCHORING OF IRON PARTICLES BY FORCED HYDROLYSIS AND ITS USE TO REMOVE
Orientation: Sustainable Processes
Biography: Born in San Andres Tuxtla, Veracruz on October 27, 1990; son of Juan
Valentin Ruiz and Consuelo Reyes Cruz.
Education: Graduated from Universidad Autónoma de Nuevo León with a Bachelor
degree in Chemical Engineering in 2012.
Professional Experience: Quality Engineer in Walker Bay Boats from 2011 to 2012.
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universidad autónoma de nuevo león faculty of chemical sciences

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